diff --git a/lib/_debugger.js b/lib/_debugger.js
index 0dc9c95c2c8e13..955080ff2fe8cd 100644
--- a/lib/_debugger.js
+++ b/lib/_debugger.js
@@ -164,7 +164,8 @@ exports.Client = Client;
 
 
 Client.prototype._addHandle = function(desc) {
-  if (!util.isObject(desc) || !util.isNumber(desc.handle)) {
+  if (desc === null || typeof desc !== 'object' ||
+      typeof desc.handle !== 'number') {
     return;
   }
 
@@ -282,7 +283,7 @@ Client.prototype.reqLookup = function(refs, cb) {
   this.req(req, function(err, res) {
     if (err) return cb(err);
     for (var ref in res) {
-      if (util.isObject(res[ref])) {
+      if (res[ref] !== null && typeof res[ref] === 'object') {
         self._addHandle(res[ref]);
       }
     }
@@ -544,8 +545,7 @@ Client.prototype.mirrorObject = function(handle, depth, cb) {
           mirrorValue = '[?]';
         }
 
-
-        if (util.isArray(mirror) && !util.isNumber(prop.name)) {
+        if (Array.isArray(mirror) && typeof prop.name !== 'number') {
           // Skip the 'length' property.
           return;
         }
@@ -578,7 +578,7 @@ Client.prototype.mirrorObject = function(handle, depth, cb) {
     val = function() {};
   } else if (handle.type === 'null') {
     val = null;
-  } else if (!util.isUndefined(handle.value)) {
+  } else if (handle.value !== undefined) {
     val = handle.value;
   } else if (handle.type === 'undefined') {
     val = undefined;
@@ -879,7 +879,7 @@ Interface.prototype.print = function(text, oneline) {
   if (this.killed) return;
   this.clearline();
 
-  this.stdout.write(util.isString(text) ? text : util.inspect(text));
+  this.stdout.write(typeof text === 'string' ? text : util.inspect(text));
 
   if (oneline !== true) {
     this.stdout.write('\n');
@@ -1194,7 +1194,7 @@ Interface.prototype.scripts = function() {
   this.pause();
   for (var id in client.scripts) {
     var script = client.scripts[id];
-    if (util.isObject(script) && script.name) {
+    if (script !== null && typeof script === 'object' && script.name) {
       if (displayNatives ||
           script.name == client.currentScript ||
           !script.isNative) {
@@ -1332,13 +1332,13 @@ Interface.prototype.setBreakpoint = function(script, line,
       ambiguous;
 
   // setBreakpoint() should insert breakpoint on current line
-  if (util.isUndefined(script)) {
+  if (script === undefined) {
     script = this.client.currentScript;
     line = this.client.currentSourceLine + 1;
   }
 
   // setBreakpoint(line-number) should insert breakpoint in current script
-  if (util.isUndefined(line) && util.isNumber(script)) {
+  if (line === undefined && typeof script === 'number') {
     line = script;
     script = this.client.currentScript;
   }
@@ -1442,7 +1442,7 @@ Interface.prototype.clearBreakpoint = function(script, line) {
     if (bp.scriptId === script ||
         bp.scriptReq === script ||
         (bp.script && bp.script.indexOf(script) !== -1)) {
-      if (!util.isUndefined(index)) {
+      if (index !== undefined) {
         ambiguous = true;
       }
       scriptId = script;
@@ -1470,11 +1470,11 @@ Interface.prototype.clearBreakpoint = function(script, line) {
 
   if (ambiguous) return this.error('Script name is ambiguous');
 
-  if (util.isUndefined(scriptId)) {
+  if (scriptId === undefined) {
     return this.error('Script ' + script + ' not found');
   }
 
-  if (util.isUndefined(breakpoint)) {
+  if (breakpoint === undefined) {
     return this.error('Breakpoint not found on line ' + line);
   }
 
diff --git a/lib/_http_client.js b/lib/_http_client.js
index 9212c0a1bada1a..6804157ee0a4dc 100644
--- a/lib/_http_client.js
+++ b/lib/_http_client.js
@@ -19,7 +19,7 @@ function ClientRequest(options, cb) {
   var self = this;
   OutgoingMessage.call(self);
 
-  if (util.isString(options)) {
+  if (typeof options === 'string') {
     options = url.parse(options);
   }
 
@@ -27,7 +27,8 @@ function ClientRequest(options, cb) {
   var defaultAgent = options._defaultAgent || Agent.globalAgent;
   if (agent === false) {
     agent = new defaultAgent.constructor();
-  } else if (util.isNullOrUndefined(agent) && !options.createConnection) {
+  } else if ((agent === null || agent === undefined) &&
+             !options.createConnection) {
     agent = defaultAgent;
   }
   self.agent = agent;
@@ -56,7 +57,7 @@ function ClientRequest(options, cb) {
   var port = options.port = options.port || defaultPort || 80;
   var host = options.host = options.hostname || options.host || 'localhost';
 
-  if (util.isUndefined(options.setHost)) {
+  if (options.setHost === undefined) {
     var setHost = true;
   }
 
@@ -68,7 +69,7 @@ function ClientRequest(options, cb) {
     self.once('response', cb);
   }
 
-  if (!util.isArray(options.headers)) {
+  if (!Array.isArray(options.headers)) {
     if (options.headers) {
       var keys = Object.keys(options.headers);
       for (var i = 0, l = keys.length; i < l; i++) {
@@ -101,7 +102,7 @@ function ClientRequest(options, cb) {
     self.useChunkedEncodingByDefault = true;
   }
 
-  if (util.isArray(options.headers)) {
+  if (Array.isArray(options.headers)) {
     self._storeHeader(self.method + ' ' + self.path + ' HTTP/1.1\r\n',
                       options.headers);
   } else if (self.getHeader('expect')) {
@@ -446,7 +447,7 @@ function tickOnSocket(req, socket) {
   httpSocketSetup(socket);
 
   // Propagate headers limit from request object to parser
-  if (util.isNumber(req.maxHeadersCount)) {
+  if (typeof req.maxHeadersCount === 'number') {
     parser.maxHeaderPairs = req.maxHeadersCount << 1;
   } else {
     // Set default value because parser may be reused from FreeList
diff --git a/lib/_http_common.js b/lib/_http_common.js
index 5f5c2d7fb2001f..209bd77bf452cc 100644
--- a/lib/_http_common.js
+++ b/lib/_http_common.js
@@ -8,7 +8,6 @@ const IncomingMessage = incoming.IncomingMessage;
 const readStart = incoming.readStart;
 const readStop = incoming.readStop;
 
-const isNumber = require('util').isNumber;
 const debug = require('util').debuglog('http');
 exports.debug = debug;
 
@@ -69,7 +68,7 @@ function parserOnHeadersComplete(versionMajor, versionMinor, headers, method,
 
   parser.incoming._addHeaderLines(headers, n);
 
-  if (isNumber(method)) {
+  if (typeof method === 'number') {
     // server only
     parser.incoming.method = HTTPParser.methods[method];
   } else {
diff --git a/lib/_http_incoming.js b/lib/_http_incoming.js
index e35457ce5eb6c7..41ece371cdfbe6 100644
--- a/lib/_http_incoming.js
+++ b/lib/_http_incoming.js
@@ -130,7 +130,7 @@ IncomingMessage.prototype._addHeaderLine = function(field, value, dest) {
   switch (field) {
     // Array headers:
     case 'set-cookie':
-      if (!util.isUndefined(dest[field])) {
+      if (dest[field] !== undefined) {
         dest[field].push(value);
       } else {
         dest[field] = [value];
@@ -152,13 +152,13 @@ IncomingMessage.prototype._addHeaderLine = function(field, value, dest) {
     case 'location':
     case 'max-forwards':
       // drop duplicates
-      if (util.isUndefined(dest[field]))
+      if (dest[field] === undefined)
         dest[field] = value;
       break;
 
     default:
       // make comma-separated list
-      if (!util.isUndefined(dest[field]))
+      if (dest[field] !== undefined)
         dest[field] += ', ' + value;
       else {
         dest[field] = value;
diff --git a/lib/_http_outgoing.js b/lib/_http_outgoing.js
index a93b7e0683c986..25157d34a44298 100644
--- a/lib/_http_outgoing.js
+++ b/lib/_http_outgoing.js
@@ -106,7 +106,7 @@ OutgoingMessage.prototype._send = function(data, encoding, callback) {
   // the same packet. Future versions of Node are going to take care of
   // this at a lower level and in a more general way.
   if (!this._headerSent) {
-    if (util.isString(data) &&
+    if (typeof data === 'string' &&
         encoding !== 'hex' &&
         encoding !== 'base64') {
       data = this._header + data;
@@ -122,13 +122,13 @@ OutgoingMessage.prototype._send = function(data, encoding, callback) {
 
 
 OutgoingMessage.prototype._writeRaw = function(data, encoding, callback) {
-  if (util.isFunction(encoding)) {
+  if (typeof encoding === 'function') {
     callback = encoding;
     encoding = null;
   }
 
   if (data.length === 0) {
-    if (util.isFunction(callback))
+    if (typeof callback === 'function')
       process.nextTick(callback);
     return true;
   }
@@ -187,7 +187,7 @@ OutgoingMessage.prototype._storeHeader = function(firstLine, headers) {
 
   if (headers) {
     var keys = Object.keys(headers);
-    var isArray = util.isArray(headers);
+    var isArray = Array.isArray(headers);
     var field, value;
 
     for (var i = 0, l = keys.length; i < l; i++) {
@@ -200,7 +200,7 @@ OutgoingMessage.prototype._storeHeader = function(firstLine, headers) {
         value = headers[key];
       }
 
-      if (util.isArray(value)) {
+      if (Array.isArray(value)) {
         for (var j = 0; j < value.length; j++) {
           storeHeader(this, state, field, value[j]);
         }
@@ -408,7 +408,7 @@ OutgoingMessage.prototype.write = function(chunk, encoding, callback) {
     return true;
   }
 
-  if (!util.isString(chunk) && !util.isBuffer(chunk)) {
+  if (typeof chunk !== 'string' && !(chunk instanceof Buffer)) {
     throw new TypeError('first argument must be a string or Buffer');
   }
 
@@ -419,7 +419,7 @@ OutgoingMessage.prototype.write = function(chunk, encoding, callback) {
 
   var len, ret;
   if (this.chunkedEncoding) {
-    if (util.isString(chunk) &&
+    if (typeof chunk === 'string' &&
         encoding !== 'hex' &&
         encoding !== 'base64' &&
         encoding !== 'binary') {
@@ -428,7 +428,7 @@ OutgoingMessage.prototype.write = function(chunk, encoding, callback) {
       ret = this._send(chunk, encoding, callback);
     } else {
       // buffer, or a non-toString-friendly encoding
-      if (util.isString(chunk))
+      if (typeof chunk === 'string')
         len = Buffer.byteLength(chunk, encoding);
       else
         len = chunk.length;
@@ -458,7 +458,7 @@ OutgoingMessage.prototype.write = function(chunk, encoding, callback) {
 OutgoingMessage.prototype.addTrailers = function(headers) {
   this._trailer = '';
   var keys = Object.keys(headers);
-  var isArray = util.isArray(headers);
+  var isArray = Array.isArray(headers);
   var field, value;
   for (var i = 0, l = keys.length; i < l; i++) {
     var key = keys[i];
@@ -479,15 +479,15 @@ const crlf_buf = new Buffer('\r\n');
 
 
 OutgoingMessage.prototype.end = function(data, encoding, callback) {
-  if (util.isFunction(data)) {
+  if (typeof data === 'function') {
     callback = data;
     data = null;
-  } else if (util.isFunction(encoding)) {
+  } else if (typeof encoding === 'function') {
     callback = encoding;
     encoding = null;
   }
 
-  if (data && !util.isString(data) && !util.isBuffer(data)) {
+  if (data && typeof data !== 'string' && !(data instanceof Buffer)) {
     throw new TypeError('first argument must be a string or Buffer');
   }
 
@@ -500,10 +500,9 @@ OutgoingMessage.prototype.end = function(data, encoding, callback) {
     self.emit('finish');
   }
 
-  if (util.isFunction(callback))
+  if (typeof callback === 'function')
     this.once('finish', callback);
 
-
   if (!this._header) {
     this._implicitHeader();
   }
diff --git a/lib/_http_server.js b/lib/_http_server.js
index 20012a5ab1c870..d8b979943841e1 100644
--- a/lib/_http_server.js
+++ b/lib/_http_server.js
@@ -153,7 +153,7 @@ ServerResponse.prototype._implicitHeader = function() {
 ServerResponse.prototype.writeHead = function(statusCode, reason, obj) {
   var headers;
 
-  if (util.isString(reason)) {
+  if (typeof reason === 'string') {
     // writeHead(statusCode, reasonPhrase[, headers])
     this.statusMessage = reason;
   } else {
@@ -297,7 +297,7 @@ function connectionListener(socket) {
   parser.incoming = null;
 
   // Propagate headers limit from server instance to parser
-  if (util.isNumber(this.maxHeadersCount)) {
+  if (typeof this.maxHeadersCount === 'number') {
     parser.maxHeaderPairs = this.maxHeadersCount << 1;
   } else {
     // Set default value because parser may be reused from FreeList
@@ -455,7 +455,7 @@ function connectionListener(socket) {
       }
     }
 
-    if (!util.isUndefined(req.headers.expect) &&
+    if (req.headers.expect !== undefined &&
         (req.httpVersionMajor == 1 && req.httpVersionMinor == 1) &&
         continueExpression.test(req.headers['expect'])) {
       res._expect_continue = true;
diff --git a/lib/_stream_readable.js b/lib/_stream_readable.js
index da164208289d1f..2418648b69caa0 100644
--- a/lib/_stream_readable.js
+++ b/lib/_stream_readable.js
@@ -95,7 +95,7 @@ function Readable(options) {
 Readable.prototype.push = function(chunk, encoding) {
   var state = this._readableState;
 
-  if (util.isString(chunk) && !state.objectMode) {
+  if (!state.objectMode && typeof chunk === 'string') {
     encoding = encoding || state.defaultEncoding;
     if (encoding !== state.encoding) {
       chunk = new Buffer(chunk, encoding);
@@ -209,7 +209,7 @@ function howMuchToRead(n, state) {
   if (state.objectMode)
     return n === 0 ? 0 : 1;
 
-  if (util.isNull(n) || isNaN(n)) {
+  if (n === null || isNaN(n)) {
     // only flow one buffer at a time
     if (state.flowing && state.buffer.length)
       return state.buffer[0].length;
@@ -245,7 +245,7 @@ Readable.prototype.read = function(n) {
   var state = this._readableState;
   var nOrig = n;
 
-  if (!util.isNumber(n) || n > 0)
+  if (typeof n !== 'number' || n > 0)
     state.emittedReadable = false;
 
   // if we're doing read(0) to trigger a readable event, but we
@@ -333,7 +333,7 @@ Readable.prototype.read = function(n) {
   else
     ret = null;
 
-  if (util.isNull(ret)) {
+  if (ret === null) {
     state.needReadable = true;
     n = 0;
   }
@@ -349,7 +349,7 @@ Readable.prototype.read = function(n) {
   if (nOrig !== n && state.ended && state.length === 0)
     endReadable(this);
 
-  if (!util.isNull(ret))
+  if (ret !== null)
     this.emit('data', ret);
 
   return ret;
@@ -357,9 +357,10 @@ Readable.prototype.read = function(n) {
 
 function chunkInvalid(state, chunk) {
   var er = null;
-  if (!util.isBuffer(chunk) &&
-      !util.isString(chunk) &&
-      !util.isNullOrUndefined(chunk) &&
+  if (!(chunk instanceof Buffer) &&
+      typeof chunk !== 'string' &&
+      chunk !== null &&
+      chunk !== undefined &&
       !state.objectMode) {
     er = new TypeError('Invalid non-string/buffer chunk');
   }
@@ -757,7 +758,6 @@ Readable.prototype.wrap = function(stream) {
       chunk = state.decoder.write(chunk);
 
     // don't skip over falsy values in objectMode
-    //if (state.objectMode && util.isNullOrUndefined(chunk))
     if (state.objectMode && (chunk === null || chunk === undefined))
       return;
     else if (!state.objectMode && (!chunk || !chunk.length))
@@ -773,7 +773,7 @@ Readable.prototype.wrap = function(stream) {
   // proxy all the other methods.
   // important when wrapping filters and duplexes.
   for (var i in stream) {
-    if (util.isFunction(stream[i]) && util.isUndefined(this[i])) {
+    if (this[i] === undefined && typeof stream[i] === 'function') {
       this[i] = function(method) { return function() {
         return stream[method].apply(stream, arguments);
       }}(i);
diff --git a/lib/_stream_transform.js b/lib/_stream_transform.js
index f83620863308dc..d3e7a1348634ce 100644
--- a/lib/_stream_transform.js
+++ b/lib/_stream_transform.js
@@ -72,7 +72,7 @@ function afterTransform(stream, er, data) {
   ts.writechunk = null;
   ts.writecb = null;
 
-  if (!util.isNullOrUndefined(data))
+  if (data !== null && data !== undefined)
     stream.push(data);
 
   if (cb)
@@ -106,7 +106,7 @@ function Transform(options) {
   this._readableState.sync = false;
 
   this.once('prefinish', function() {
-    if (util.isFunction(this._flush))
+    if (typeof this._flush === 'function')
       this._flush(function(er) {
         done(stream, er);
       });
@@ -154,7 +154,7 @@ Transform.prototype._write = function(chunk, encoding, cb) {
 Transform.prototype._read = function(n) {
   var ts = this._transformState;
 
-  if (!util.isNull(ts.writechunk) && ts.writecb && !ts.transforming) {
+  if (ts.writechunk !== null && ts.writecb && !ts.transforming) {
     ts.transforming = true;
     this._transform(ts.writechunk, ts.writeencoding, ts.afterTransform);
   } else {
diff --git a/lib/_stream_writable.js b/lib/_stream_writable.js
index a3c65f45951800..0176f4095fc76e 100644
--- a/lib/_stream_writable.js
+++ b/lib/_stream_writable.js
@@ -162,9 +162,11 @@ function writeAfterEnd(stream, state, cb) {
 // how many bytes or characters.
 function validChunk(stream, state, chunk, cb) {
   var valid = true;
-  if (!util.isBuffer(chunk) &&
-      !util.isString(chunk) &&
-      !util.isNullOrUndefined(chunk) &&
+
+  if (!(chunk instanceof Buffer) &&
+      typeof chunk !== 'string' &&
+      chunk !== null &&
+      chunk !== undefined &&
       !state.objectMode) {
     var er = new TypeError('Invalid non-string/buffer chunk');
     stream.emit('error', er);
@@ -180,17 +182,17 @@ Writable.prototype.write = function(chunk, encoding, cb) {
   var state = this._writableState;
   var ret = false;
 
-  if (util.isFunction(encoding)) {
+  if (typeof encoding === 'function') {
     cb = encoding;
     encoding = null;
   }
 
-  if (util.isBuffer(chunk))
+  if (chunk instanceof Buffer)
     encoding = 'buffer';
   else if (!encoding)
     encoding = state.defaultEncoding;
 
-  if (!util.isFunction(cb))
+  if (typeof cb !== 'function')
     cb = nop;
 
   if (state.ended)
@@ -236,7 +238,7 @@ Writable.prototype.setDefaultEncoding = function setDefaultEncoding(encoding) {
 function decodeChunk(state, chunk, encoding) {
   if (!state.objectMode &&
       state.decodeStrings !== false &&
-      util.isString(chunk)) {
+      typeof chunk === 'string') {
     chunk = new Buffer(chunk, encoding);
   }
   return chunk;
@@ -247,7 +249,8 @@ function decodeChunk(state, chunk, encoding) {
 // If we return false, then we need a drain event, so set that flag.
 function writeOrBuffer(stream, state, chunk, encoding, cb) {
   chunk = decodeChunk(state, chunk, encoding);
-  if (util.isBuffer(chunk))
+
+  if (chunk instanceof Buffer)
     encoding = 'buffer';
   var len = state.objectMode ? 1 : chunk.length;
 
@@ -418,16 +421,16 @@ Writable.prototype._writev = null;
 Writable.prototype.end = function(chunk, encoding, cb) {
   var state = this._writableState;
 
-  if (util.isFunction(chunk)) {
+  if (typeof chunk === 'function') {
     cb = chunk;
     chunk = null;
     encoding = null;
-  } else if (util.isFunction(encoding)) {
+  } else if (typeof encoding === 'function') {
     cb = encoding;
     encoding = null;
   }
 
-  if (!util.isNullOrUndefined(chunk))
+  if (chunk !== null && chunk !== undefined)
     this.write(chunk, encoding);
 
   // .end() fully uncorks
diff --git a/lib/_tls_common.js b/lib/_tls_common.js
index 3db0d6969fcb65..2c15d91df8d4c3 100644
--- a/lib/_tls_common.js
+++ b/lib/_tls_common.js
@@ -1,6 +1,5 @@
 'use strict';
 
-const util = require('util');
 const constants = require('constants');
 const tls = require('tls');
 
@@ -47,7 +46,7 @@ exports.createSecureContext = function createSecureContext(options, context) {
   // NOTE: It's important to add CA before the cert to be able to load
   // cert's issuer in C++ code.
   if (options.ca) {
-    if (util.isArray(options.ca)) {
+    if (Array.isArray(options.ca)) {
       for (var i = 0, len = options.ca.length; i < len; i++) {
         c.context.addCACert(options.ca[i]);
       }
@@ -95,7 +94,7 @@ exports.createSecureContext = function createSecureContext(options, context) {
   else
     c.context.setCiphers(tls.DEFAULT_CIPHERS);
 
-  if (util.isUndefined(options.ecdhCurve))
+  if (options.ecdhCurve === undefined)
     c.context.setECDHCurve(tls.DEFAULT_ECDH_CURVE);
   else if (options.ecdhCurve)
     c.context.setECDHCurve(options.ecdhCurve);
@@ -103,7 +102,7 @@ exports.createSecureContext = function createSecureContext(options, context) {
   if (options.dhparam) c.context.setDHParam(options.dhparam);
 
   if (options.crl) {
-    if (util.isArray(options.crl)) {
+    if (Array.isArray(options.crl)) {
       for (var i = 0, len = options.crl.length; i < len; i++) {
         c.context.addCRL(options.crl[i]);
       }
diff --git a/lib/_tls_legacy.js b/lib/_tls_legacy.js
index f5be838d0549b2..4148085503fc64 100644
--- a/lib/_tls_legacy.js
+++ b/lib/_tls_legacy.js
@@ -36,7 +36,7 @@ SlabBuffer.prototype.use = function use(context, fn, size) {
 
   var actualSize = this.remaining;
 
-  if (!util.isNull(size)) actualSize = Math.min(size, actualSize);
+  if (size !== null) actualSize = Math.min(size, actualSize);
 
   var bytes = fn.call(context, this.pool, this.offset, actualSize);
   if (bytes > 0) {
@@ -72,7 +72,7 @@ function CryptoStream(pair, options) {
   this._finished = false;
   this._opposite = null;
 
-  if (util.isNull(slabBuffer)) slabBuffer = new SlabBuffer();
+  if (slabBuffer === null) slabBuffer = new SlabBuffer();
   this._buffer = slabBuffer;
 
   this.once('finish', onCryptoStreamFinish);
@@ -142,7 +142,7 @@ CryptoStream.prototype.init = function init() {
 
 
 CryptoStream.prototype._write = function write(data, encoding, cb) {
-  assert(util.isNull(this._pending));
+  assert(this._pending === null);
 
   // Black-hole data
   if (!this.pair.ssl) return cb(null);
@@ -189,7 +189,7 @@ CryptoStream.prototype._write = function write(data, encoding, cb) {
 
       // Invoke callback only when all data read from opposite stream
       if (this._opposite._halfRead) {
-        assert(util.isNull(this._sslOutCb));
+        assert(this._sslOutCb === null);
         this._sslOutCb = cb;
       } else {
         cb(null);
@@ -288,8 +288,8 @@ CryptoStream.prototype._read = function read(size) {
   }
 
   // Try writing pending data
-  if (!util.isNull(this._pending)) this._writePending();
-  if (!util.isNull(this._opposite._pending)) this._opposite._writePending();
+  if (this._pending !== null) this._writePending();
+  if (this._opposite._pending !== null) this._opposite._writePending();
 
   if (bytesRead === 0) {
     // EOF when cleartext has finished and we have nothing to read
@@ -398,7 +398,7 @@ CryptoStream.prototype.end = function(chunk, encoding) {
   }
 
   // Write pending data first
-  if (!util.isNull(this._pending)) this._writePending();
+  if (this._pending !== null) this._writePending();
 
   this.writable = false;
 
diff --git a/lib/_tls_wrap.js b/lib/_tls_wrap.js
index 6ce7c9f6bac327..c6bf31b98095ad 100644
--- a/lib/_tls_wrap.js
+++ b/lib/_tls_wrap.js
@@ -447,7 +447,7 @@ TLSSocket.prototype.setServername = function(name) {
 };
 
 TLSSocket.prototype.setSession = function(session) {
-  if (util.isString(session))
+  if (typeof session === 'string')
     session = new Buffer(session, 'binary');
   this.ssl.setSession(session);
 };
@@ -554,10 +554,11 @@ TLSSocket.prototype.getCipher = function(err) {
 //
 function Server(/* [options], listener */) {
   var options, listener;
-  if (util.isObject(arguments[0])) {
+
+  if (arguments[0] !== null && typeof arguments[0] === 'object') {
     options = arguments[0];
     listener = arguments[1];
-  } else if (util.isFunction(arguments[0])) {
+  } else if (typeof arguments[0] === 'function') {
     options = {};
     listener = arguments[0];
   }
@@ -590,7 +591,7 @@ function Server(/* [options], listener */) {
 
   var timeout = options.handshakeTimeout || (120 * 1000);
 
-  if (!util.isNumber(timeout)) {
+  if (typeof timeout !== 'number') {
     throw new TypeError('handshakeTimeout must be a number');
   }
 
@@ -676,13 +677,13 @@ Server.prototype._setServerData = function(data) {
 
 
 Server.prototype.setOptions = function(options) {
-  if (util.isBoolean(options.requestCert)) {
+  if (typeof options.requestCert === 'boolean') {
     this.requestCert = options.requestCert;
   } else {
     this.requestCert = false;
   }
 
-  if (util.isBoolean(options.rejectUnauthorized)) {
+  if (typeof options.rejectUnauthorized === 'boolean') {
     this.rejectUnauthorized = options.rejectUnauthorized;
   } else {
     this.rejectUnauthorized = false;
@@ -696,7 +697,7 @@ Server.prototype.setOptions = function(options) {
   if (options.secureProtocol) this.secureProtocol = options.secureProtocol;
   if (options.crl) this.crl = options.crl;
   if (options.ciphers) this.ciphers = options.ciphers;
-  if (!util.isUndefined(options.ecdhCurve))
+  if (options.ecdhCurve !== undefined)
     this.ecdhCurve = options.ecdhCurve;
   if (options.dhparam) this.dhparam = options.dhparam;
   if (options.sessionTimeout) this.sessionTimeout = options.sessionTimeout;
@@ -734,7 +735,7 @@ function SNICallback(servername, callback) {
   var ctx;
 
   this.server._contexts.some(function(elem) {
-    if (!util.isNull(servername.match(elem[0]))) {
+    if (servername.match(elem[0]) !== null) {
       ctx = elem[1];
       return true;
     }
@@ -763,9 +764,9 @@ function normalizeConnectArgs(listArgs) {
   var options = args[0];
   var cb = args[1];
 
-  if (util.isObject(listArgs[1])) {
+  if (listArgs[1] !== null && typeof listArgs[1] === 'object') {
     options = util._extend(options, listArgs[1]);
-  } else if (util.isObject(listArgs[2])) {
+  } else if (listArgs[2] !== null && typeof listArgs[2] === 'object') {
     options = util._extend(options, listArgs[2]);
   }
 
diff --git a/lib/assert.js b/lib/assert.js
index a13cc503163c33..5cd75b220a2448 100644
--- a/lib/assert.js
+++ b/lib/assert.js
@@ -59,7 +59,7 @@ assert.AssertionError = function AssertionError(options) {
 util.inherits(assert.AssertionError, Error);
 
 function truncate(s, n) {
-  if (util.isString(s)) {
+  if (typeof s === 'string') {
     return s.length < n ? s : s.slice(0, n);
   } else {
     return s;
@@ -138,8 +138,7 @@ function _deepEqual(actual, expected) {
   // 7.1. All identical values are equivalent, as determined by ===.
   if (actual === expected) {
     return true;
-
-  } else if (util.isBuffer(actual) && util.isBuffer(expected)) {
+  } else if (actual instanceof Buffer && expected instanceof Buffer) {
     if (actual.length != expected.length) return false;
 
     for (var i = 0; i < actual.length; i++) {
@@ -165,7 +164,8 @@ function _deepEqual(actual, expected) {
 
   // 7.4. Other pairs that do not both pass typeof value == 'object',
   // equivalence is determined by ==.
-  } else if (!util.isObject(actual) && !util.isObject(expected)) {
+  } else if ((actual === null || typeof actual !== 'object') &&
+             (expected === null || typeof expected !== 'object')) {
     return actual == expected;
 
   // 7.5 For all other Object pairs, including Array objects, equivalence is
@@ -184,7 +184,7 @@ function isArguments(object) {
 }
 
 function objEquiv(a, b) {
-  if (util.isNullOrUndefined(a) || util.isNullOrUndefined(b))
+  if (a === null || a === undefined || b === null || b === undefined)
     return false;
   // an identical 'prototype' property.
   if (a.prototype !== b.prototype) return false;
@@ -270,11 +270,11 @@ function expectedException(actual, expected) {
 function _throws(shouldThrow, block, expected, message) {
   var actual;
 
-  if (!util.isFunction(block)) {
+  if (typeof block !== 'function') {
     throw new TypeError('block must be a function');
   }
 
-  if (util.isString(expected)) {
+  if (typeof expected === 'string') {
     message = expected;
     expected = null;
   }
diff --git a/lib/buffer.js b/lib/buffer.js
index 41de85e6733fba..19208b36ab6561 100644
--- a/lib/buffer.js
+++ b/lib/buffer.js
@@ -27,20 +27,20 @@ createPool();
 
 
 function Buffer(subject, encoding) {
-  if (!util.isBuffer(this))
+  if (!(this instanceof Buffer))
     return new Buffer(subject, encoding);
 
-  if (util.isNumber(subject)) {
+  if (typeof subject === 'number') {
     this.length = +subject;
 
-  } else if (util.isString(subject)) {
-    if (!util.isString(encoding) || encoding.length === 0)
+  } else if (typeof subject === 'string') {
+    if (typeof encoding !== 'string' || encoding.length === 0)
       encoding = 'utf8';
     this.length = Buffer.byteLength(subject, encoding);
 
   // Handle Arrays, Buffers, Uint8Arrays or JSON.
-  } else if (util.isObject(subject)) {
-    if (subject.type === 'Buffer' && util.isArray(subject.data))
+  } else if (subject !== null && typeof subject === 'object') {
+    if (subject.type === 'Buffer' && Array.isArray(subject.data))
       subject = subject.data;
     this.length = +subject.length;
 
@@ -71,11 +71,11 @@ function Buffer(subject, encoding) {
     alloc(this, this.length);
   }
 
-  if (util.isNumber(subject)) {
+  if (typeof subject === 'number') {
     return;
   }
 
-  if (util.isString(subject)) {
+  if (typeof subject === 'string') {
     // In the case of base64 it's possible that the size of the buffer
     // allocated was slightly too large. In this case we need to rewrite
     // the length to the actual length written.
@@ -88,10 +88,10 @@ function Buffer(subject, encoding) {
       poolOffset -= (prevLen - len);
     }
 
-  } else if (util.isBuffer(subject)) {
+  } else if (subject instanceof Buffer) {
     subject.copy(this, 0, 0, this.length);
 
-  } else if (util.isNumber(subject.length) || util.isArray(subject)) {
+  } else if (typeof subject.length === 'number' || Array.isArray(subject)) {
     // Really crappy way to handle Uint8Arrays, but V8 doesn't give a simple
     // way to access the data from the C++ API.
     for (var i = 0; i < this.length; i++)
@@ -130,7 +130,7 @@ buffer.setupBufferJS(NativeBuffer, internal);
 // Static methods
 
 Buffer.isBuffer = function isBuffer(b) {
-  return util.isBuffer(b);
+  return b instanceof Buffer;
 };
 
 
@@ -165,10 +165,10 @@ Buffer.isEncoding = function(encoding) {
 
 
 Buffer.concat = function(list, length) {
-  if (!util.isArray(list))
+  if (!Array.isArray(list))
     throw new TypeError('Usage: Buffer.concat(list[, length])');
 
-  if (util.isUndefined(length)) {
+  if (length === undefined) {
     length = 0;
     for (var i = 0; i < list.length; i++)
       length += list[i].length;
@@ -223,7 +223,7 @@ Buffer.prototype.toString = function(encoding, start, end) {
   var loweredCase = false;
 
   start = start >>> 0;
-  end = util.isUndefined(end) || end === Infinity ? this.length : end >>> 0;
+  end = end === undefined || end === Infinity ? this.length : end >>> 0;
 
   if (!encoding) encoding = 'utf8';
   if (start < 0) start = 0;
@@ -341,13 +341,13 @@ const writeMsg = '.write(string, encoding, offset, length) is deprecated.' +
                  ' Use write(string[, offset[, length]][, encoding]) instead.';
 Buffer.prototype.write = function(string, offset, length, encoding) {
   // Buffer#write(string);
-  if (util.isUndefined(offset)) {
+  if (offset === undefined) {
     encoding = 'utf8';
     length = this.length;
     offset = 0;
 
   // Buffer#write(string, encoding)
-  } else if (util.isUndefined(length) && util.isString(offset)) {
+  } else if (length === undefined && typeof offset === 'string') {
     encoding = offset;
     length = this.length;
     offset = 0;
@@ -357,7 +357,7 @@ Buffer.prototype.write = function(string, offset, length, encoding) {
     offset = offset >>> 0;
     if (isFinite(length)) {
       length = length >>> 0;
-      if (util.isUndefined(encoding))
+      if (encoding === undefined)
         encoding = 'utf8';
     } else {
       encoding = length;
@@ -383,7 +383,7 @@ Buffer.prototype.write = function(string, offset, length, encoding) {
   }
 
   var remaining = this.length - offset;
-  if (util.isUndefined(length) || length > remaining)
+  if (length === undefined || length > remaining)
     length = remaining;
 
   encoding = !!encoding ? (encoding + '').toLowerCase() : 'utf8';
@@ -443,7 +443,7 @@ Buffer.prototype.toJSON = function() {
 Buffer.prototype.slice = function(start, end) {
   var len = this.length;
   start = ~~start;
-  end = util.isUndefined(end) ? len : ~~end;
+  end = end === undefined ? len : ~~end;
 
   if (start < 0) {
     start += len;
@@ -468,7 +468,7 @@ Buffer.prototype.slice = function(start, end) {
   sliceOnto(this, buf, start, end);
   buf.length = end - start;
   if (buf.length > 0)
-    buf.parent = util.isUndefined(this.parent) ? this : this.parent;
+    buf.parent = this.parent === undefined ? this : this.parent;
 
   return buf;
 };
diff --git a/lib/child_process.js b/lib/child_process.js
index 9226d127b3b064..48f95e50c0709a 100644
--- a/lib/child_process.js
+++ b/lib/child_process.js
@@ -22,7 +22,7 @@ function handleWrapGetter(name, callback) {
 
   Object.defineProperty(handleWraps, name, {
     get: function() {
-      if (!util.isUndefined(cons)) return cons;
+      if (cons !== undefined) return cons;
       return cons = callback();
     }
   });
@@ -295,9 +295,9 @@ function getSocketList(type, slave, key) {
 const INTERNAL_PREFIX = 'NODE_';
 function handleMessage(target, message, handle) {
   var eventName = 'message';
-  if (!util.isNull(message) &&
-      util.isObject(message) &&
-      util.isString(message.cmd) &&
+  if (message !== null &&
+      typeof message === 'object' &&
+      typeof message.cmd === 'string' &&
       message.cmd.length > INTERNAL_PREFIX.length &&
       message.cmd.slice(0, INTERNAL_PREFIX.length) === INTERNAL_PREFIX) {
     eventName = 'internalMessage';
@@ -353,7 +353,7 @@ function setupChannel(target, channel) {
   target.on('internalMessage', function(message, handle) {
     // Once acknowledged - continue sending handles.
     if (message.cmd === 'NODE_HANDLE_ACK') {
-      assert(util.isArray(target._handleQueue));
+      assert(Array.isArray(target._handleQueue));
       var queue = target._handleQueue;
       target._handleQueue = null;
 
@@ -400,7 +400,7 @@ function setupChannel(target, channel) {
   target._send = function(message, handle, swallowErrors) {
     assert(this.connected || this._channel);
 
-    if (util.isUndefined(message))
+    if (message === undefined)
       throw new TypeError('message cannot be undefined');
 
     // package messages with a handle object
@@ -538,7 +538,7 @@ exports.fork = function(modulePath /*, args, options*/) {
 
   // Get options and args arguments.
   var options, args, execArgv;
-  if (util.isArray(arguments[1])) {
+  if (Array.isArray(arguments[1])) {
     args = arguments[1];
     options = util._extend({}, arguments[2]);
   } else {
@@ -583,7 +583,7 @@ exports._forkChild = function(fd) {
 function normalizeExecArgs(command /*, options, callback */) {
   var file, args, options, callback;
 
-  if (util.isFunction(arguments[1])) {
+  if (typeof arguments[1] === 'function') {
     options = undefined;
     callback = arguments[1];
   } else {
@@ -638,11 +638,11 @@ exports.execFile = function(file /* args, options, callback */) {
 
   // Parse the parameters.
 
-  if (util.isFunction(arguments[arguments.length - 1])) {
+  if (typeof arguments[arguments.length - 1] === 'function') {
     callback = arguments[arguments.length - 1];
   }
 
-  if (util.isArray(arguments[1])) {
+  if (Array.isArray(arguments[1])) {
     args = arguments[1];
     options = util._extend(options, arguments[2]);
   } else {
@@ -806,14 +806,14 @@ function _validateStdio(stdio, sync) {
       ipcFd;
 
   // Replace shortcut with an array
-  if (util.isString(stdio)) {
+  if (typeof stdio === 'string') {
     switch (stdio) {
       case 'ignore': stdio = ['ignore', 'ignore', 'ignore']; break;
       case 'pipe': stdio = ['pipe', 'pipe', 'pipe']; break;
       case 'inherit': stdio = [0, 1, 2]; break;
       default: throw new TypeError('Incorrect value of stdio option: ' + stdio);
     }
-  } else if (!util.isArray(stdio)) {
+  } else if (!Array.isArray(stdio)) {
     throw new TypeError('Incorrect value of stdio option: ' +
         util.inspect(stdio));
   }
@@ -837,13 +837,13 @@ function _validateStdio(stdio, sync) {
     }
 
     // Defaults
-    if (util.isNullOrUndefined(stdio)) {
+    if (stdio === null || stdio === undefined) {
       stdio = i < 3 ? 'pipe' : 'ignore';
     }
 
     if (stdio === null || stdio === 'ignore') {
       acc.push({type: 'ignore'});
-    } else if (stdio === 'pipe' || util.isNumber(stdio) && stdio < 0) {
+    } else if (stdio === 'pipe' || typeof stdio === 'number' && stdio < 0) {
       var a = {
         type: 'pipe',
         readable: i === 0,
@@ -855,7 +855,7 @@ function _validateStdio(stdio, sync) {
 
       acc.push(a);
     } else if (stdio === 'ipc') {
-      if (sync || !util.isUndefined(ipc)) {
+      if (sync || ipc !== undefined) {
         // Cleanup previously created pipes
         cleanup();
         if (!sync)
@@ -877,7 +877,7 @@ function _validateStdio(stdio, sync) {
         type: 'inherit',
         fd: i
       });
-    } else if (util.isNumber(stdio) || util.isNumber(stdio.fd)) {
+    } else if (typeof stdio === 'number' || typeof stdio.fd === 'number') {
       acc.push({
         type: 'fd',
         fd: stdio.fd || stdio
@@ -893,7 +893,7 @@ function _validateStdio(stdio, sync) {
         wrapType: getHandleWrapType(handle),
         handle: handle
       });
-    } else if (util.isBuffer(stdio) || util.isString(stdio)) {
+    } else if ((stdio instanceof Buffer) || typeof stdio === 'string') {
       if (!sync) {
         cleanup();
         throw new TypeError('Asynchronous forks do not support Buffer input: ' +
@@ -919,7 +919,8 @@ function normalizeSpawnArguments(file /*, args, options*/) {
   if (Array.isArray(arguments[1])) {
     args = arguments[1].slice(0);
     options = arguments[2];
-  } else if (arguments[1] !== undefined && !util.isObject(arguments[1])) {
+  } else if (arguments[1] !== undefined &&
+             (arguments[1] === null || typeof arguments[1] !== 'object')) {
     throw new TypeError('Incorrect value of args option');
   } else {
     args = [];
@@ -928,7 +929,7 @@ function normalizeSpawnArguments(file /*, args, options*/) {
 
   if (options === undefined)
     options = {};
-  else if (!util.isObject(options))
+  else if (options === null || typeof options !== 'object')
     throw new TypeError('options argument must be an object');
 
   options = util._extend({}, options);
@@ -1089,7 +1090,7 @@ ChildProcess.prototype.spawn = function(options) {
   ipcFd = stdio.ipcFd;
   stdio = options.stdio = stdio.stdio;
 
-  if (!util.isUndefined(ipc)) {
+  if (ipc !== undefined) {
     // Let child process know about opened IPC channel
     options.envPairs = options.envPairs || [];
     options.envPairs.push('NODE_CHANNEL_FD=' + ipcFd);
@@ -1150,19 +1151,19 @@ ChildProcess.prototype.spawn = function(options) {
     }
   });
 
-  this.stdin = stdio.length >= 1 && !util.isUndefined(stdio[0].socket) ?
+  this.stdin = stdio.length >= 1 && stdio[0].socket !== undefined ?
       stdio[0].socket : null;
-  this.stdout = stdio.length >= 2 && !util.isUndefined(stdio[1].socket) ?
+  this.stdout = stdio.length >= 2 && stdio[1].socket !== undefined ?
       stdio[1].socket : null;
-  this.stderr = stdio.length >= 3 && !util.isUndefined(stdio[2].socket) ?
+  this.stderr = stdio.length >= 3 && stdio[2].socket !== undefined ?
       stdio[2].socket : null;
 
   this.stdio = stdio.map(function(stdio) {
-    return util.isUndefined(stdio.socket) ? null : stdio.socket;
+    return stdio.socket === undefined ? null : stdio.socket;
   });
 
   // Add .send() method and start listening for IPC data
-  if (!util.isUndefined(ipc)) setupChannel(this, ipc);
+  if (ipc !== undefined) setupChannel(this, ipc);
 
   return err;
 };
@@ -1183,7 +1184,7 @@ ChildProcess.prototype.kill = function(sig) {
     signal = constants[sig];
   }
 
-  if (util.isUndefined(signal)) {
+  if (signal === undefined) {
     throw new Error('Unknown signal: ' + sig);
   }
 
@@ -1262,7 +1263,7 @@ function spawnSync(/*file, args, options*/) {
       var pipe = options.stdio[i] = util._extend({}, options.stdio[i]);
       if (Buffer.isBuffer(input))
         pipe.input = input;
-      else if (util.isString(input))
+      else if (typeof input === 'string')
         pipe.input = new Buffer(input, options.encoding);
       else
         throw new TypeError(util.format(
diff --git a/lib/cluster.js b/lib/cluster.js
index 9312fc28f28fcf..14d0bc69b3ba19 100644
--- a/lib/cluster.js
+++ b/lib/cluster.js
@@ -22,7 +22,7 @@ function Worker(options) {
 
   EventEmitter.call(this);
 
-  if (!util.isObject(options))
+  if (options === null || typeof options !== 'object')
     options = {};
 
   this.suicide = undefined;
@@ -68,7 +68,7 @@ function SharedHandle(key, address, port, addressType, backlog, fd) {
   else
     rval = net._createServerHandle(address, port, addressType, fd);
 
-  if (util.isNumber(rval))
+  if (typeof rval === 'number')
     this.errno = rval;
   else
     this.handle = rval;
@@ -135,7 +135,7 @@ RoundRobinHandle.prototype.add = function(worker, send) {
     self.handoff(worker);  // In case there are connections pending.
   }
 
-  if (util.isNull(this.server)) return done();
+  if (this.server === null) return done();
   // Still busy binding.
   this.server.once('listening', done);
   this.server.once('error', function(err) {
@@ -170,7 +170,7 @@ RoundRobinHandle.prototype.handoff = function(worker) {
     return;  // Worker is closing (or has closed) the server.
   }
   var handle = this.handles.shift();
-  if (util.isUndefined(handle)) {
+  if (handle === undefined) {
     this.free.push(worker);  // Add to ready queue again.
     return;
   }
@@ -203,7 +203,7 @@ function masterInit() {
     'rr': SCHED_RR
   }[process.env.NODE_CLUSTER_SCHED_POLICY];
 
-  if (util.isUndefined(schedulingPolicy)) {
+  if (schedulingPolicy === undefined) {
     // FIXME Round-robin doesn't perform well on Windows right now due to the
     // way IOCP is wired up. Bert is going to fix that, eventually.
     schedulingPolicy = (process.platform === 'win32') ? SCHED_NONE : SCHED_RR;
@@ -438,7 +438,7 @@ function masterInit() {
                 message.fd];
     var key = args.join(':');
     var handle = handles[key];
-    if (util.isUndefined(handle)) {
+    if (handle === undefined) {
       var constructor = RoundRobinHandle;
       // UDP is exempt from round-robin connection balancing for what should
       // be obvious reasons: it's connectionless. There is nothing to send to
@@ -562,7 +562,7 @@ function workerInit() {
       delete handles[key];
       return close.apply(this, arguments);
     };
-    assert(util.isUndefined(handles[key]));
+    assert(handles[key] === undefined);
     handles[key] = handle;
     cb(message.errno, handle);
   }
@@ -586,7 +586,7 @@ function workerInit() {
       // the ack by the master process in which we can still receive handles.
       // onconnection() below handles that by sending those handles back to
       // the master.
-      if (util.isUndefined(key)) return;
+      if (key === undefined) return;
       send({ act: 'close', key: key });
       delete handles[key];
       key = undefined;
@@ -607,7 +607,7 @@ function workerInit() {
     if (message.sockname) {
       handle.getsockname = getsockname;  // TCP handles only.
     }
-    assert(util.isUndefined(handles[key]));
+    assert(handles[key] === undefined);
     handles[key] = handle;
     cb(0, handle);
   }
@@ -616,7 +616,7 @@ function workerInit() {
   function onconnection(message, handle) {
     var key = message.key;
     var server = handles[key];
-    var accepted = !util.isUndefined(server);
+    var accepted = server !== undefined;
     send({ ack: message.seq, accepted: accepted });
     if (accepted) server.onconnection(0, handle);
   }
@@ -664,7 +664,7 @@ function internal(worker, cb) {
   return function(message, handle) {
     if (message.cmd !== 'NODE_CLUSTER') return;
     var fn = cb;
-    if (!util.isUndefined(message.ack)) {
+    if (message.ack !== undefined) {
       fn = callbacks[message.ack];
       delete callbacks[message.ack];
     }
diff --git a/lib/console.js b/lib/console.js
index 323d23374a8653..63e3c5e0bf1aa4 100644
--- a/lib/console.js
+++ b/lib/console.js
@@ -6,7 +6,7 @@ function Console(stdout, stderr) {
   if (!(this instanceof Console)) {
     return new Console(stdout, stderr);
   }
-  if (!stdout || !util.isFunction(stdout.write)) {
+  if (!stdout || typeof stdout.write !== 'function') {
     throw new TypeError('Console expects a writable stream instance');
   }
   if (!stderr) {
diff --git a/lib/crypto.js b/lib/crypto.js
index f8efffda483ec7..42564c79e47226 100644
--- a/lib/crypto.js
+++ b/lib/crypto.js
@@ -25,7 +25,7 @@ const DH_GENERATOR = 2;
 // to break them unnecessarily.
 function toBuf(str, encoding) {
   encoding = encoding || 'binary';
-  if (util.isString(str)) {
+  if (typeof str === 'string') {
     if (encoding === 'buffer')
       encoding = 'binary';
     str = new Buffer(str, encoding);
@@ -92,7 +92,7 @@ Hash.prototype._flush = function(callback) {
 
 Hash.prototype.update = function(data, encoding) {
   encoding = encoding || exports.DEFAULT_ENCODING;
-  if (encoding === 'buffer' && util.isString(data))
+  if (encoding === 'buffer' && typeof data === 'string')
     encoding = 'binary';
   this._handle.update(data, encoding);
   return this;
@@ -369,7 +369,7 @@ function DiffieHellman(sizeOrKey, keyEncoding, generator, genEncoding) {
   if (!(this instanceof DiffieHellman))
     return new DiffieHellman(sizeOrKey, keyEncoding, generator, genEncoding);
 
-  if (!util.isBuffer(sizeOrKey) &&
+  if (!(sizeOrKey instanceof Buffer) &&
       typeof sizeOrKey !== 'number' &&
       typeof sizeOrKey !== 'string')
     throw new TypeError('First argument should be number, string or Buffer');
@@ -513,7 +513,7 @@ DiffieHellman.prototype.setPrivateKey = function(key, encoding) {
 
 
 function ECDH(curve) {
-  if (!util.isString(curve))
+  if (typeof curve !== 'string')
     throw new TypeError('curve should be a string');
 
   this._handle = new binding.ECDH(curve);
@@ -566,12 +566,12 @@ exports.pbkdf2 = function(password,
                           keylen,
                           digest,
                           callback) {
-  if (util.isFunction(digest)) {
+  if (typeof digest === 'function') {
     callback = digest;
     digest = undefined;
   }
 
-  if (!util.isFunction(callback))
+  if (typeof callback !== 'function')
     throw new Error('No callback provided to pbkdf2');
 
   return pbkdf2(password, salt, iterations, keylen, digest, callback);
@@ -632,10 +632,10 @@ Certificate.prototype.exportChallenge = function(object, encoding) {
 
 
 exports.setEngine = function setEngine(id, flags) {
-  if (!util.isString(id))
+  if (typeof id !== 'string')
     throw new TypeError('id should be a string');
 
-  if (flags && !util.isNumber(flags))
+  if (flags && typeof flags !== 'number')
     throw new TypeError('flags should be a number, if present');
   flags = flags >>> 0;
 
diff --git a/lib/dgram.js b/lib/dgram.js
index 19cf1c4ea00c53..a919e16497486e 100644
--- a/lib/dgram.js
+++ b/lib/dgram.js
@@ -61,7 +61,7 @@ function newHandle(type) {
 
 exports._createSocketHandle = function(address, port, addressType, fd) {
   // Opening an existing fd is not supported for UDP handles.
-  assert(!util.isNumber(fd) || fd < 0);
+  assert(typeof fd !== 'number' || fd < 0);
 
   var handle = newHandle(addressType);
 
@@ -97,7 +97,7 @@ function Socket(type, listener) {
   // If true - UV_UDP_REUSEADDR flag will be set
   this._reuseAddr = options && options.reuseAddr;
 
-  if (util.isFunction(listener))
+  if (typeof listener === 'function')
     this.on('message', listener);
 }
 util.inherits(Socket, events.EventEmitter);
@@ -143,7 +143,7 @@ Socket.prototype.bind = function(port /*, address, callback*/) {
 
   this._bindState = BIND_STATE_BINDING;
 
-  if (util.isFunction(arguments[arguments.length - 1]))
+  if (typeof arguments[arguments.length - 1] === 'function')
     self.once('listening', arguments[arguments.length - 1]);
 
   const UDP = process.binding('udp_wrap').UDP;
@@ -156,12 +156,12 @@ Socket.prototype.bind = function(port /*, address, callback*/) {
   var address;
   var exclusive;
 
-  if (util.isObject(port)) {
+  if (port !== null && typeof port === 'object') {
     address = port.address || '';
     exclusive = !!port.exclusive;
     port = port.port;
   } else {
-    address = util.isFunction(arguments[1]) ? '' : arguments[1];
+    address = typeof arguments[1] === 'function' ? '' : arguments[1];
     exclusive = false;
   }
 
@@ -225,10 +225,10 @@ Socket.prototype.sendto = function(buffer,
                                    port,
                                    address,
                                    callback) {
-  if (!util.isNumber(offset) || !util.isNumber(length))
+  if (typeof offset !== 'number' || typeof length !== 'number')
     throw new Error('send takes offset and length as args 2 and 3');
 
-  if (!util.isString(address))
+  if (typeof address !== 'string')
     throw new Error(this.type + ' sockets must send to port, address');
 
   this.send(buffer, offset, length, port, address, callback);
@@ -243,10 +243,10 @@ Socket.prototype.send = function(buffer,
                                  callback) {
   var self = this;
 
-  if (util.isString(buffer))
+  if (typeof buffer === 'string')
     buffer = new Buffer(buffer);
 
-  if (!util.isBuffer(buffer))
+  if (!(buffer instanceof Buffer))
     throw new TypeError('First argument must be a buffer or string.');
 
   offset = offset | 0;
@@ -272,7 +272,7 @@ Socket.prototype.send = function(buffer,
 
   // Normalize callback so it's either a function or undefined but not anything
   // else.
-  if (!util.isFunction(callback))
+  if (typeof callback !== 'function')
     callback = undefined;
 
   self._healthCheck();
@@ -378,7 +378,7 @@ Socket.prototype.setBroadcast = function(arg) {
 
 
 Socket.prototype.setTTL = function(arg) {
-  if (!util.isNumber(arg)) {
+  if (typeof arg !== 'number') {
     throw new TypeError('Argument must be a number');
   }
 
@@ -392,7 +392,7 @@ Socket.prototype.setTTL = function(arg) {
 
 
 Socket.prototype.setMulticastTTL = function(arg) {
-  if (!util.isNumber(arg)) {
+  if (typeof arg !== 'number') {
     throw new TypeError('Argument must be a number');
   }
 
diff --git a/lib/dns.js b/lib/dns.js
index 8cfa7f58c565a8..e084fb5e5d48a8 100644
--- a/lib/dns.js
+++ b/lib/dns.js
@@ -53,7 +53,7 @@ function errnoException(err, syscall, hostname) {
 //   callback.immediately = true;
 // }
 function makeAsync(callback) {
-  if (!util.isFunction(callback)) {
+  if (typeof callback !== 'function') {
     return callback;
   }
   return function asyncCallback() {
@@ -96,7 +96,7 @@ exports.lookup = function lookup(hostname, options, callback) {
     family = 0;
   } else if (typeof callback !== 'function') {
     throw TypeError('invalid arguments: callback must be passed');
-  } else if (util.isObject(options)) {
+  } else if (options !== null && typeof options === 'object') {
     hints = options.hints >>> 0;
     family = options.family >>> 0;
 
@@ -192,9 +192,9 @@ function resolver(bindingName) {
   var binding = cares[bindingName];
 
   return function query(name, callback) {
-    if (!util.isString(name)) {
+    if (typeof name !== 'string') {
       throw new Error('Name must be a string');
-    } else if (!util.isFunction(callback)) {
+    } else if (typeof callback !== 'function') {
       throw new Error('Callback must be a function');
     }
 
@@ -228,17 +228,17 @@ exports.reverse = resolveMap.PTR = resolver('getHostByAddr');
 
 exports.resolve = function(hostname, type_, callback_) {
   var resolver, callback;
-  if (util.isString(type_)) {
+  if (typeof type_ === 'string') {
     resolver = resolveMap[type_];
     callback = callback_;
-  } else if (util.isFunction(type_)) {
+  } else if (typeof type_ === 'function') {
     resolver = exports.resolve4;
     callback = type_;
   } else {
     throw new Error('Type must be a string');
   }
 
-  if (util.isFunction(resolver)) {
+  if (typeof resolver === 'function') {
     return resolver(hostname, callback);
   } else {
     throw new Error('Unknown type "' + type_ + '"');
diff --git a/lib/events.js b/lib/events.js
index 672131deedb6ef..cece093d3876e8 100644
--- a/lib/events.js
+++ b/lib/events.js
@@ -1,7 +1,6 @@
 'use strict';
 
 var domain;
-const util = require('util');
 
 function EventEmitter() {
   EventEmitter.init.call(this);
@@ -40,14 +39,14 @@ EventEmitter.init = function() {
 // Obviously not all Emitters should be limited to 10. This function allows
 // that to be increased. Set to zero for unlimited.
 EventEmitter.prototype.setMaxListeners = function setMaxListeners(n) {
-  if (!util.isNumber(n) || n < 0 || isNaN(n))
+  if (typeof n !== 'number' || n < 0 || isNaN(n))
     throw TypeError('n must be a positive number');
   this._maxListeners = n;
   return this;
 };
 
 function $getMaxListeners(that) {
-  if (util.isUndefined(that._maxListeners))
+  if (that._maxListeners === undefined)
     return EventEmitter.defaultMaxListeners;
   return that._maxListeners;
 }
@@ -82,13 +81,13 @@ EventEmitter.prototype.emit = function emit(type) {
 
   handler = this._events[type];
 
-  if (util.isUndefined(handler))
+  if (handler === undefined)
     return false;
 
   if (this.domain && this !== process)
     this.domain.enter();
 
-  if (util.isFunction(handler)) {
+  if (typeof handler === 'function') {
     switch (arguments.length) {
       // fast cases
       case 1:
@@ -108,7 +107,7 @@ EventEmitter.prototype.emit = function emit(type) {
           args[i - 1] = arguments[i];
         handler.apply(this, args);
     }
-  } else if (util.isObject(handler)) {
+  } else if (handler !== null && typeof handler === 'object') {
     len = arguments.length;
     args = new Array(len - 1);
     for (i = 1; i < len; i++)
@@ -129,7 +128,7 @@ EventEmitter.prototype.emit = function emit(type) {
 EventEmitter.prototype.addListener = function addListener(type, listener) {
   var m;
 
-  if (!util.isFunction(listener))
+  if (typeof listener !== 'function')
     throw TypeError('listener must be a function');
 
   if (!this._events)
@@ -139,13 +138,13 @@ EventEmitter.prototype.addListener = function addListener(type, listener) {
   // adding it to the listeners, first emit "newListener".
   if (this._events.newListener)
     this.emit('newListener', type,
-              util.isFunction(listener.listener) ?
+              typeof listener.listener === 'function' ?
               listener.listener : listener);
 
   if (!this._events[type])
     // Optimize the case of one listener. Don't need the extra array object.
     this._events[type] = listener;
-  else if (util.isObject(this._events[type]))
+  else if (typeof this._events[type] === 'object')
     // If we've already got an array, just append.
     this._events[type].push(listener);
   else
@@ -153,7 +152,8 @@ EventEmitter.prototype.addListener = function addListener(type, listener) {
     this._events[type] = [this._events[type], listener];
 
   // Check for listener leak
-  if (util.isObject(this._events[type]) && !this._events[type].warned) {
+  if (this._events[type] !== null && typeof this._events[type] === 'object' &&
+      !this._events[type].warned) {
     var m = $getMaxListeners(this);
     if (m && m > 0 && this._events[type].length > m) {
       this._events[type].warned = true;
@@ -171,7 +171,7 @@ EventEmitter.prototype.addListener = function addListener(type, listener) {
 EventEmitter.prototype.on = EventEmitter.prototype.addListener;
 
 EventEmitter.prototype.once = function once(type, listener) {
-  if (!util.isFunction(listener))
+  if (typeof listener !== 'function')
     throw TypeError('listener must be a function');
 
   var fired = false;
@@ -196,7 +196,7 @@ EventEmitter.prototype.removeListener =
     function removeListener(type, listener) {
       var list, position, length, i;
 
-      if (!util.isFunction(listener))
+      if (typeof listener !== 'function')
         throw TypeError('listener must be a function');
 
       if (!this._events || !this._events[type])
@@ -207,12 +207,13 @@ EventEmitter.prototype.removeListener =
       position = -1;
 
       if (list === listener ||
-          (util.isFunction(list.listener) && list.listener === listener)) {
+          (typeof list.listener === 'function' &&
+          list.listener === listener)) {
         delete this._events[type];
         if (this._events.removeListener)
           this.emit('removeListener', type, listener);
 
-      } else if (util.isObject(list)) {
+      } else if (list !== null && typeof list === 'object') {
         for (i = length; i-- > 0;) {
           if (list[i] === listener ||
               (list[i].listener && list[i].listener === listener)) {
@@ -267,7 +268,7 @@ EventEmitter.prototype.removeAllListeners =
 
       listeners = this._events[type];
 
-      if (util.isFunction(listeners)) {
+      if (typeof listeners === 'function') {
         this.removeListener(type, listeners);
       } else if (Array.isArray(listeners)) {
         // LIFO order
@@ -283,7 +284,7 @@ EventEmitter.prototype.listeners = function listeners(type) {
   var ret;
   if (!this._events || !this._events[type])
     ret = [];
-  else if (util.isFunction(this._events[type]))
+  else if (typeof this._events[type] === 'function')
     ret = [this._events[type]];
   else
     ret = this._events[type].slice();
@@ -294,7 +295,7 @@ EventEmitter.listenerCount = function(emitter, type) {
   var ret;
   if (!emitter._events || !emitter._events[type])
     ret = 0;
-  else if (util.isFunction(emitter._events[type]))
+  else if (typeof emitter._events[type] === 'function')
     ret = 1;
   else
     ret = emitter._events[type].length;
diff --git a/lib/fs.js b/lib/fs.js
index d7c82f9dbc927d..d967d22f59da8d 100644
--- a/lib/fs.js
+++ b/lib/fs.js
@@ -57,14 +57,14 @@ function rethrow() {
 }
 
 function maybeCallback(cb) {
-  return util.isFunction(cb) ? cb : rethrow();
+  return typeof cb === 'function' ? cb : rethrow();
 }
 
 // Ensure that callbacks run in the global context. Only use this function
 // for callbacks that are passed to the binding layer, callbacks that are
 // invoked from JS already run in the proper scope.
 function makeCallback(cb) {
-  if (!util.isFunction(cb)) {
+  if (typeof cb !== 'function') {
     return rethrow();
   }
 
@@ -218,11 +218,11 @@ fs.existsSync = function(path) {
 fs.readFile = function(path, options, callback_) {
   var callback = maybeCallback(arguments[arguments.length - 1]);
 
-  if (util.isFunction(options) || !options) {
+  if (!options || typeof options === 'function') {
     options = { encoding: null, flag: 'r' };
-  } else if (util.isString(options)) {
+  } else if (typeof options === 'string') {
     options = { encoding: options, flag: 'r' };
-  } else if (!util.isObject(options)) {
+  } else if (typeof options !== 'object') {
     throw new TypeError('Bad arguments');
   }
 
@@ -317,9 +317,9 @@ fs.readFile = function(path, options, callback_) {
 fs.readFileSync = function(path, options) {
   if (!options) {
     options = { encoding: null, flag: 'r' };
-  } else if (util.isString(options)) {
+  } else if (typeof options === 'string') {
     options = { encoding: options, flag: 'r' };
-  } else if (!util.isObject(options)) {
+  } else if (typeof options !== 'object') {
     throw new TypeError('Bad arguments');
   }
 
@@ -395,7 +395,7 @@ fs.readFileSync = function(path, options) {
 // Used by binding.open and friends
 function stringToFlags(flag) {
   // Only mess with strings
-  if (!util.isString(flag)) {
+  if (typeof flag !== 'string') {
     return flag;
   }
 
@@ -448,9 +448,9 @@ fs.closeSync = function(fd) {
 };
 
 function modeNum(m, def) {
-  if (util.isNumber(m))
+  if (typeof m === 'number')
     return m;
-  if (util.isString(m))
+  if (typeof m === 'string')
     return parseInt(m, 8);
   if (def)
     return modeNum(def);
@@ -479,7 +479,7 @@ fs.openSync = function(path, flags, mode) {
 };
 
 fs.read = function(fd, buffer, offset, length, position, callback) {
-  if (!util.isBuffer(buffer)) {
+  if (!(buffer instanceof Buffer)) {
     // legacy string interface (fd, length, position, encoding, callback)
     var cb = arguments[4],
         encoding = arguments[3];
@@ -513,7 +513,7 @@ fs.read = function(fd, buffer, offset, length, position, callback) {
 
 fs.readSync = function(fd, buffer, offset, length, position) {
   var legacy = false;
-  if (!util.isBuffer(buffer)) {
+  if (!(buffer instanceof Buffer)) {
     // legacy string interface (fd, length, position, encoding, callback)
     legacy = true;
     var encoding = arguments[3];
@@ -551,9 +551,9 @@ fs.write = function(fd, buffer, offset, length, position, callback) {
     callback(err, written || 0, buffer);
   }
 
-  if (util.isBuffer(buffer)) {
+  if (buffer instanceof Buffer) {
     // if no position is passed then assume null
-    if (util.isFunction(position)) {
+    if (typeof position === 'function') {
       callback = position;
       position = null;
     }
@@ -563,10 +563,10 @@ fs.write = function(fd, buffer, offset, length, position, callback) {
     return binding.writeBuffer(fd, buffer, offset, length, position, req);
   }
 
-  if (util.isString(buffer))
+  if (typeof buffer === 'string')
     buffer += '';
-  if (!util.isFunction(position)) {
-    if (util.isFunction(offset)) {
+  if (typeof position !== 'function') {
+    if (typeof offset === 'function') {
       position = offset;
       offset = null;
     } else {
@@ -585,14 +585,14 @@ fs.write = function(fd, buffer, offset, length, position, callback) {
 // OR
 //  fs.writeSync(fd, string[, position[, encoding]]);
 fs.writeSync = function(fd, buffer, offset, length, position) {
-  if (util.isBuffer(buffer)) {
-    if (util.isUndefined(position))
+  if (buffer instanceof Buffer) {
+    if (position === undefined)
       position = null;
     return binding.writeBuffer(fd, buffer, offset, length, position);
   }
-  if (!util.isString(buffer))
+  if (typeof buffer !== 'string')
     buffer += '';
-  if (util.isUndefined(offset))
+  if (offset === undefined)
     offset = null;
   return binding.writeString(fd, buffer, offset, length, position);
 };
@@ -616,15 +616,15 @@ fs.renameSync = function(oldPath, newPath) {
 };
 
 fs.truncate = function(path, len, callback) {
-  if (util.isNumber(path)) {
+  if (typeof path === 'number') {
     var req = new FSReqWrap();
     req.oncomplete = callback;
     return fs.ftruncate(path, len, req);
   }
-  if (util.isFunction(len)) {
+  if (typeof len === 'function') {
     callback = len;
     len = 0;
-  } else if (util.isUndefined(len)) {
+  } else if (len === undefined) {
     len = 0;
   }
 
@@ -642,11 +642,11 @@ fs.truncate = function(path, len, callback) {
 };
 
 fs.truncateSync = function(path, len) {
-  if (util.isNumber(path)) {
+  if (typeof path === 'number') {
     // legacy
     return fs.ftruncateSync(path, len);
   }
-  if (util.isUndefined(len)) {
+  if (len === undefined) {
     len = 0;
   }
   // allow error to be thrown, but still close fd.
@@ -660,10 +660,10 @@ fs.truncateSync = function(path, len) {
 };
 
 fs.ftruncate = function(fd, len, callback) {
-  if (util.isFunction(len)) {
+  if (typeof len === 'function') {
     callback = len;
     len = 0;
-  } else if (util.isUndefined(len)) {
+  } else if (len === undefined) {
     len = 0;
   }
   var req = new FSReqWrap();
@@ -672,7 +672,7 @@ fs.ftruncate = function(fd, len, callback) {
 };
 
 fs.ftruncateSync = function(fd, len) {
-  if (util.isUndefined(len)) {
+  if (len === undefined) {
     len = 0;
   }
   return binding.ftruncate(fd, len);
@@ -712,7 +712,7 @@ fs.fsyncSync = function(fd) {
 };
 
 fs.mkdir = function(path, mode, callback) {
-  if (util.isFunction(mode)) callback = mode;
+  if (typeof mode === 'function') callback = mode;
   callback = makeCallback(callback);
   if (!nullCheck(path, callback)) return;
   var req = new FSReqWrap();
@@ -806,7 +806,7 @@ function preprocessSymlinkDestination(path, type, linkPath) {
 }
 
 fs.symlink = function(destination, path, type_, callback) {
-  var type = (util.isString(type_) ? type_ : null);
+  var type = (typeof type_ === 'string' ? type_ : null);
   var callback = makeCallback(arguments[arguments.length - 1]);
 
   if (!nullCheck(destination, callback)) return;
@@ -822,7 +822,7 @@ fs.symlink = function(destination, path, type_, callback) {
 };
 
 fs.symlinkSync = function(destination, path, type) {
-  type = (util.isString(type) ? type : null);
+  type = (typeof type === 'string' ? type : null);
 
   nullCheck(destination);
   nullCheck(path);
@@ -973,7 +973,7 @@ fs.chownSync = function(path, uid, gid) {
 
 // converts Date or number to a fractional UNIX timestamp
 function toUnixTimestamp(time) {
-  if (util.isNumber(time)) {
+  if (typeof time === 'number') {
     return time;
   }
   if (util.isDate(time)) {
@@ -1043,11 +1043,11 @@ function writeAll(fd, buffer, offset, length, position, callback) {
 fs.writeFile = function(path, data, options, callback) {
   var callback = maybeCallback(arguments[arguments.length - 1]);
 
-  if (util.isFunction(options) || !options) {
+  if (!options || typeof options === 'function') {
     options = { encoding: 'utf8', mode: 0o666, flag: 'w' };
-  } else if (util.isString(options)) {
+  } else if (typeof options === 'string') {
     options = { encoding: options, mode: 0o666, flag: 'w' };
-  } else if (!util.isObject(options)) {
+  } else if (typeof options !== 'object') {
     throw new TypeError('Bad arguments');
   }
 
@@ -1058,7 +1058,7 @@ fs.writeFile = function(path, data, options, callback) {
     if (openErr) {
       if (callback) callback(openErr);
     } else {
-      var buffer = util.isBuffer(data) ? data : new Buffer('' + data,
+      var buffer = (data instanceof Buffer) ? data : new Buffer('' + data,
           options.encoding || 'utf8');
       var position = /a/.test(flag) ? null : 0;
       writeAll(fd, buffer, 0, buffer.length, position, callback);
@@ -1069,9 +1069,9 @@ fs.writeFile = function(path, data, options, callback) {
 fs.writeFileSync = function(path, data, options) {
   if (!options) {
     options = { encoding: 'utf8', mode: 0o666, flag: 'w' };
-  } else if (util.isString(options)) {
+  } else if (typeof options === 'string') {
     options = { encoding: options, mode: 0o666, flag: 'w' };
-  } else if (!util.isObject(options)) {
+  } else if (typeof options !== 'object') {
     throw new TypeError('Bad arguments');
   }
 
@@ -1079,7 +1079,7 @@ fs.writeFileSync = function(path, data, options) {
 
   var flag = options.flag || 'w';
   var fd = fs.openSync(path, flag, options.mode);
-  if (!util.isBuffer(data)) {
+  if (!(data instanceof Buffer)) {
     data = new Buffer('' + data, options.encoding || 'utf8');
   }
   var written = 0;
@@ -1098,11 +1098,11 @@ fs.writeFileSync = function(path, data, options) {
 fs.appendFile = function(path, data, options, callback_) {
   var callback = maybeCallback(arguments[arguments.length - 1]);
 
-  if (util.isFunction(options) || !options) {
+  if (!options || typeof options === 'function') {
     options = { encoding: 'utf8', mode: 0o666, flag: 'a' };
-  } else if (util.isString(options)) {
+  } else if (typeof options === 'string') {
     options = { encoding: options, mode: 0o666, flag: 'a' };
-  } else if (!util.isObject(options)) {
+  } else if (typeof options !== 'object') {
     throw new TypeError('Bad arguments');
   }
 
@@ -1114,9 +1114,9 @@ fs.appendFile = function(path, data, options, callback_) {
 fs.appendFileSync = function(path, data, options) {
   if (!options) {
     options = { encoding: 'utf8', mode: 0o666, flag: 'a' };
-  } else if (util.isString(options)) {
+  } else if (typeof options === 'string') {
     options = { encoding: options, mode: 0o666, flag: 'a' };
-  } else if (!util.isObject(options)) {
+  } else if (typeof options !== 'object') {
     throw new TypeError('Bad arguments');
   }
   if (!options.flag)
@@ -1165,7 +1165,7 @@ fs.watch = function(filename) {
   var options;
   var listener;
 
-  if (util.isObject(arguments[1])) {
+  if (arguments[1] !== null && typeof arguments[1] === 'object') {
     options = arguments[1];
     listener = arguments[2];
   } else {
@@ -1173,8 +1173,8 @@ fs.watch = function(filename) {
     listener = arguments[1];
   }
 
-  if (util.isUndefined(options.persistent)) options.persistent = true;
-  if (util.isUndefined(options.recursive)) options.recursive = false;
+  if (options.persistent === undefined) options.persistent = true;
+  if (options.recursive === undefined) options.recursive = false;
 
   watcher = new FSWatcher();
   watcher.start(filename, options.persistent, options.recursive);
@@ -1247,7 +1247,7 @@ fs.watchFile = function(filename) {
     persistent: true
   };
 
-  if (util.isObject(arguments[1])) {
+  if (arguments[1] !== null && typeof arguments[1] === 'object') {
     options = util._extend(options, arguments[1]);
     listener = arguments[2];
   } else {
@@ -1275,7 +1275,7 @@ fs.unwatchFile = function(filename, listener) {
 
   var stat = statWatchers[filename];
 
-  if (util.isFunction(listener)) {
+  if (typeof listener === 'function') {
     stat.removeListener('change', listener);
   } else {
     stat.removeAllListeners('change');
@@ -1378,7 +1378,7 @@ fs.realpathSync = function realpathSync(p, cache) {
           linkTarget = seenLinks[id];
         }
       }
-      if (util.isNull(linkTarget)) {
+      if (linkTarget === null) {
         fs.statSync(base);
         linkTarget = fs.readlinkSync(base);
       }
@@ -1400,7 +1400,7 @@ fs.realpathSync = function realpathSync(p, cache) {
 
 
 fs.realpath = function realpath(p, cache, cb) {
-  if (!util.isFunction(cb)) {
+  if (typeof cb !== 'function') {
     cb = maybeCallback(cache);
     cache = null;
   }
@@ -1561,13 +1561,13 @@ function ReadStream(path, options) {
       options.autoClose : true;
   this.pos = undefined;
 
-  if (!util.isUndefined(this.start)) {
-    if (!util.isNumber(this.start)) {
+  if (this.start !== undefined) {
+    if (typeof this.start !== 'number') {
       throw TypeError('start must be a Number');
     }
-    if (util.isUndefined(this.end)) {
+    if (this.end === undefined) {
       this.end = Infinity;
-    } else if (!util.isNumber(this.end)) {
+    } else if (typeof this.end !== 'number') {
       throw TypeError('end must be a Number');
     }
 
@@ -1578,7 +1578,7 @@ function ReadStream(path, options) {
     this.pos = this.start;
   }
 
-  if (!util.isNumber(this.fd))
+  if (typeof this.fd !== 'number')
     this.open();
 
   this.on('end', function() {
@@ -1609,7 +1609,7 @@ ReadStream.prototype.open = function() {
 };
 
 ReadStream.prototype._read = function(n) {
-  if (!util.isNumber(this.fd))
+  if (typeof this.fd !== 'number')
     return this.once('open', function() {
       this._read(n);
     });
@@ -1630,7 +1630,7 @@ ReadStream.prototype._read = function(n) {
   var toRead = Math.min(pool.length - pool.used, n);
   var start = pool.used;
 
-  if (!util.isUndefined(this.pos))
+  if (this.pos !== undefined)
     toRead = Math.min(this.end - this.pos + 1, toRead);
 
   // already read everything we were supposed to read!
@@ -1643,7 +1643,7 @@ ReadStream.prototype._read = function(n) {
   fs.read(this.fd, pool, pool.used, toRead, this.pos, onread);
 
   // move the pool positions, and internal position for reading.
-  if (!util.isUndefined(this.pos))
+  if (this.pos !== undefined)
     this.pos += toRead;
   pool.used += toRead;
 
@@ -1676,8 +1676,8 @@ ReadStream.prototype.close = function(cb) {
   var self = this;
   if (cb)
     this.once('close', cb);
-  if (this.closed || !util.isNumber(this.fd)) {
-    if (!util.isNumber(this.fd)) {
+  if (this.closed || typeof this.fd !== 'number') {
+    if (typeof this.fd !== 'number') {
       this.once('open', close);
       return;
     }
@@ -1725,8 +1725,8 @@ function WriteStream(path, options) {
   this.pos = undefined;
   this.bytesWritten = 0;
 
-  if (!util.isUndefined(this.start)) {
-    if (!util.isNumber(this.start)) {
+  if (this.start !== undefined) {
+    if (typeof this.start !== 'number') {
       throw TypeError('start must be a Number');
     }
     if (this.start < 0) {
@@ -1736,7 +1736,7 @@ function WriteStream(path, options) {
     this.pos = this.start;
   }
 
-  if (!util.isNumber(this.fd))
+  if (typeof this.fd !== 'number')
     this.open();
 
   // dispose on finish.
@@ -1761,10 +1761,10 @@ WriteStream.prototype.open = function() {
 
 
 WriteStream.prototype._write = function(data, encoding, cb) {
-  if (!util.isBuffer(data))
+  if (!(data instanceof Buffer))
     return this.emit('error', new Error('Invalid data'));
 
-  if (!util.isNumber(this.fd))
+  if (typeof this.fd !== 'number')
     return this.once('open', function() {
       this._write(data, encoding, cb);
     });
@@ -1779,7 +1779,7 @@ WriteStream.prototype._write = function(data, encoding, cb) {
     cb();
   });
 
-  if (!util.isUndefined(this.pos))
+  if (this.pos !== undefined)
     this.pos += data.length;
 };
 
@@ -1817,10 +1817,10 @@ SyncWriteStream.prototype.write = function(data, arg1, arg2) {
 
   // parse arguments
   if (arg1) {
-    if (util.isString(arg1)) {
+    if (typeof arg1 === 'string') {
       encoding = arg1;
       cb = arg2;
-    } else if (util.isFunction(arg1)) {
+    } else if (typeof arg1 === 'function') {
       cb = arg1;
     } else {
       throw new Error('bad arg');
@@ -1829,7 +1829,7 @@ SyncWriteStream.prototype.write = function(data, arg1, arg2) {
   assertEncoding(encoding);
 
   // Change strings to buffers. SLOW
-  if (util.isString(data)) {
+  if (typeof data === 'string') {
     data = new Buffer(data, encoding);
   }
 
diff --git a/lib/https.js b/lib/https.js
index acbbe0b6c21b08..4ee15b2537421f 100644
--- a/lib/https.js
+++ b/lib/https.js
@@ -4,7 +4,7 @@ const tls = require('tls');
 const url = require('url');
 const http = require('http');
 const util = require('util');
-const inherits = require('util').inherits;
+const inherits = util.inherits;
 const debug = util.debuglog('https');
 
 function Server(opts, requestListener) {
@@ -41,21 +41,21 @@ exports.createServer = function(opts, requestListener) {
 // HTTPS agents.
 
 function createConnection(port, host, options) {
-  if (util.isObject(port)) {
+  if (port !== null && typeof port === 'object') {
     options = port;
-  } else if (util.isObject(host)) {
+  } else if (host !== null && typeof host === 'object') {
     options = host;
-  } else if (util.isObject(options)) {
+  } else if (options !== null && typeof options === 'object') {
     options = options;
   } else {
     options = {};
   }
 
-  if (util.isNumber(port)) {
+  if (typeof port === 'number') {
     options.port = port;
   }
 
-  if (util.isString(host)) {
+  if (typeof host === 'string') {
     options.host = host;
   }
 
@@ -96,7 +96,7 @@ Agent.prototype.getName = function(options) {
     name += options.pfx;
 
   name += ':';
-  if (!util.isUndefined(options.rejectUnauthorized))
+  if (options.rejectUnauthorized !== undefined)
     name += options.rejectUnauthorized;
 
   return name;
@@ -108,7 +108,7 @@ exports.globalAgent = globalAgent;
 exports.Agent = Agent;
 
 exports.request = function(options, cb) {
-  if (util.isString(options)) {
+  if (typeof options === 'string') {
     options = url.parse(options);
   } else {
     options = util._extend({}, options);
diff --git a/lib/module.js b/lib/module.js
index 07c32b7eee5b0a..3f42975a63c87a 100644
--- a/lib/module.js
+++ b/lib/module.js
@@ -347,7 +347,7 @@ Module.prototype.load = function(filename) {
 // `exports` property.
 Module.prototype.require = function(path) {
   assert(path, 'missing path');
-  assert(util.isString(path), 'path must be a string');
+  assert(typeof path === 'string', 'path must be a string');
   return Module._load(path, this);
 };
 
diff --git a/lib/net.js b/lib/net.js
index 13a3ca424bf413..1cd8b6da352789 100644
--- a/lib/net.js
+++ b/lib/net.js
@@ -45,7 +45,7 @@ function createHandle(fd) {
 const debug = util.debuglog('net');
 
 function isPipeName(s) {
-  return util.isString(s) && toNumber(s) === false;
+  return typeof s === 'string' && toNumber(s) === false;
 }
 
 exports.createServer = function() {
@@ -77,7 +77,7 @@ exports.connect = exports.createConnection = function() {
 function normalizeConnectArgs(args) {
   var options = {};
 
-  if (util.isObject(args[0])) {
+  if (args[0] !== null && typeof args[0] === 'object') {
     // connect(options, [cb])
     options = args[0];
   } else if (isPipeName(args[0])) {
@@ -86,13 +86,13 @@ function normalizeConnectArgs(args) {
   } else {
     // connect(port, [host], [cb])
     options.port = args[0];
-    if (util.isString(args[1])) {
+    if (typeof args[1] === 'string') {
       options.host = args[1];
     }
   }
 
   var cb = args[args.length - 1];
-  return util.isFunction(cb) ? [options, cb] : [options];
+  return typeof cb === 'function' ? [options, cb] : [options];
 }
 exports._normalizeConnectArgs = normalizeConnectArgs;
 
@@ -122,16 +122,16 @@ function Socket(options) {
   this._handle = null;
   this._host = null;
 
-  if (util.isNumber(options))
+  if (typeof options === 'number')
     options = { fd: options }; // Legacy interface.
-  else if (util.isUndefined(options))
+  else if (options === undefined)
     options = {};
 
   stream.Duplex.call(this, options);
 
   if (options.handle) {
     this._handle = options.handle; // private
-  } else if (!util.isUndefined(options.fd)) {
+  } else if (options.fd !== undefined) {
     this._handle = createHandle(options.fd);
     this._handle.open(options.fd);
     if ((options.fd == 1 || options.fd == 2) &&
@@ -261,7 +261,7 @@ function onSocketEnd() {
 // of the other side sending a FIN.  The standard 'write after end'
 // is overly vague, and makes it seem like the user's code is to blame.
 function writeAfterFIN(chunk, encoding, cb) {
-  if (util.isFunction(encoding)) {
+  if (typeof encoding === 'function') {
     cb = encoding;
     encoding = null;
   }
@@ -271,7 +271,7 @@ function writeAfterFIN(chunk, encoding, cb) {
   var self = this;
   // TODO: defer error events consistently everywhere, not just the cb
   self.emit('error', er);
-  if (util.isFunction(cb)) {
+  if (typeof cb === 'function') {
     process.nextTick(function() {
       cb(er);
     });
@@ -324,7 +324,7 @@ Socket.prototype._onTimeout = function() {
 Socket.prototype.setNoDelay = function(enable) {
   // backwards compatibility: assume true when `enable` is omitted
   if (this._handle && this._handle.setNoDelay)
-    this._handle.setNoDelay(util.isUndefined(enable) ? true : !!enable);
+    this._handle.setNoDelay(enable === undefined ? true : !!enable);
 };
 
 
@@ -599,7 +599,7 @@ Socket.prototype.__defineGetter__('localPort', function() {
 
 
 Socket.prototype.write = function(chunk, encoding, cb) {
-  if (!util.isString(chunk) && !util.isBuffer(chunk))
+  if (typeof chunk !== 'string' && !(chunk instanceof Buffer))
     throw new TypeError('invalid data');
   return stream.Duplex.prototype.write.apply(this, arguments);
 };
@@ -647,7 +647,7 @@ Socket.prototype._writeGeneric = function(writev, data, encoding, cb) {
     if (err === 0) req._chunks = chunks;
   } else {
     var enc;
-    if (util.isBuffer(data)) {
+    if (data instanceof Buffer) {
       req.buffer = data;  // Keep reference alive.
       enc = 'buffer';
     } else {
@@ -713,14 +713,14 @@ Socket.prototype.__defineGetter__('bytesWritten', function() {
       encoding = this._pendingEncoding;
 
   state.getBuffer().forEach(function(el) {
-    if (util.isBuffer(el.chunk))
+    if (el.chunk instanceof Buffer)
       bytes += el.chunk.length;
     else
       bytes += Buffer.byteLength(el.chunk, el.encoding);
   });
 
   if (data) {
-    if (util.isBuffer(data))
+    if (data instanceof Buffer)
       bytes += data.length;
     else
       bytes += Buffer.byteLength(data, encoding);
@@ -830,7 +830,7 @@ Socket.prototype.connect = function(options, cb) {
   if (this.write !== Socket.prototype.write)
     this.write = Socket.prototype.write;
 
-  if (!util.isObject(options)) {
+  if (options === null || typeof options !== 'object') {
     // Old API:
     // connect(port, [host], [cb])
     // connect(path, [cb]);
@@ -859,7 +859,7 @@ Socket.prototype.connect = function(options, cb) {
     initSocketHandle(this);
   }
 
-  if (util.isFunction(cb)) {
+  if (typeof cb === 'function') {
     self.once('connect', cb);
   }
 
@@ -885,7 +885,7 @@ Socket.prototype.connect = function(options, cb) {
     if (localAddress && !exports.isIP(localAddress))
       throw new TypeError('localAddress must be a valid IP: ' + localAddress);
 
-    if (localPort && !util.isNumber(localPort))
+    if (localPort && typeof localPort !== 'number')
       throw new TypeError('localPort should be a number: ' + localPort);
 
     if (port <= 0 || port > 65535)
@@ -998,13 +998,13 @@ function Server(/* [ options, ] listener */) {
 
   var options;
 
-  if (util.isFunction(arguments[0])) {
+  if (typeof arguments[0] === 'function') {
     options = {};
     self.on('connection', arguments[0]);
   } else {
     options = arguments[0] || {};
 
-    if (util.isFunction(arguments[1])) {
+    if (typeof arguments[1] === 'function') {
       self.on('connection', arguments[1]);
     }
   }
@@ -1052,7 +1052,7 @@ var createServerHandle = exports._createServerHandle =
   var handle;
 
   var isTCP = false;
-  if (util.isNumber(fd) && fd >= 0) {
+  if (typeof fd === 'number' && fd >= 0) {
     try {
       handle = createHandle(fd);
     }
@@ -1117,10 +1117,10 @@ Server.prototype._listen2 = function(address, port, addressType, backlog, fd) {
 
     var rval = null;
 
-    if (!address && !util.isNumber(fd)) {
+    if (!address && typeof fd !== 'number') {
       rval = createServerHandle('::', port, 6, fd);
 
-      if (util.isNumber(rval)) {
+      if (typeof rval === 'number') {
         rval = null;
         address = '0.0.0.0';
         addressType = 4;
@@ -1133,7 +1133,7 @@ Server.prototype._listen2 = function(address, port, addressType, backlog, fd) {
     if (rval === null)
       rval = createServerHandle(address, port, addressType, fd);
 
-    if (util.isNumber(rval)) {
+    if (typeof rval === 'number') {
       var error = exceptionWithHostPort(rval, 'listen', address, port);
       process.nextTick(function() {
         self.emit('error', error);
@@ -1212,7 +1212,7 @@ Server.prototype.listen = function() {
   var self = this;
 
   var lastArg = arguments[arguments.length - 1];
-  if (util.isFunction(lastArg)) {
+  if (typeof lastArg === 'function') {
     self.once('listening', lastArg);
   }
 
@@ -1224,24 +1224,24 @@ Server.prototype.listen = function() {
 
   const TCP = process.binding('tcp_wrap').TCP;
 
-  if (arguments.length === 0 || util.isFunction(arguments[0])) {
+  if (arguments.length === 0 || typeof arguments[0] === 'function') {
     // Bind to a random port.
     listen(self, null, 0, null, backlog);
-  } else if (util.isObject(arguments[0])) {
+  } else if (arguments[0] !== null && typeof arguments[0] === 'object') {
     var h = arguments[0];
     h = h._handle || h.handle || h;
 
     if (h instanceof TCP) {
       self._handle = h;
       listen(self, null, -1, -1, backlog);
-    } else if (util.isNumber(h.fd) && h.fd >= 0) {
+    } else if (typeof h.fd === 'number' && h.fd >= 0) {
       listen(self, null, null, null, backlog, h.fd);
     } else {
       // The first argument is a configuration object
       if (h.backlog)
         backlog = h.backlog;
 
-      if (util.isNumber(h.port)) {
+      if (typeof h.port === 'number') {
         if (h.host)
           listenAfterLookup(h.port, h.host, backlog, h.exclusive);
         else
@@ -1258,9 +1258,9 @@ Server.prototype.listen = function() {
     var pipeName = self._pipeName = arguments[0];
     listen(self, pipeName, -1, -1, backlog);
 
-  } else if (util.isUndefined(arguments[1]) ||
-             util.isFunction(arguments[1]) ||
-             util.isNumber(arguments[1])) {
+  } else if (arguments[1] === undefined ||
+             typeof arguments[1] === 'function' ||
+             typeof arguments[1] === 'number') {
     // The first argument is the port, no IP given.
     listen(self, null, port, 4, backlog);
 
@@ -1456,11 +1456,11 @@ if (process.platform === 'win32') {
   var simultaneousAccepts;
 
   exports._setSimultaneousAccepts = function(handle) {
-    if (util.isUndefined(handle)) {
+    if (handle === undefined) {
       return;
     }
 
-    if (util.isUndefined(simultaneousAccepts)) {
+    if (simultaneousAccepts === undefined) {
       simultaneousAccepts = (process.env.NODE_MANY_ACCEPTS &&
                              process.env.NODE_MANY_ACCEPTS !== '0');
     }
diff --git a/lib/path.js b/lib/path.js
index f1acb06b45a8bb..6259c5fd1d8a4a 100644
--- a/lib/path.js
+++ b/lib/path.js
@@ -1,8 +1,6 @@
 'use strict';
 
 const isWindows = process.platform === 'win32';
-const util = require('util');
-
 
 // resolves . and .. elements in a path array with directory names there
 // must be no slashes or device names (c:\) in the array
@@ -87,7 +85,7 @@ win32.resolve = function() {
     }
 
     // Skip empty and invalid entries
-    if (!util.isString(path)) {
+    if (typeof path !== 'string') {
       throw new TypeError('Arguments to path.resolve must be strings');
     } else if (!path) {
       continue;
@@ -176,7 +174,7 @@ win32.isAbsolute = function(path) {
 
 win32.join = function() {
   function f(p) {
-    if (!util.isString(p)) {
+    if (typeof p !== 'string') {
       throw new TypeError('Arguments to path.join must be strings');
     }
     return p;
@@ -265,7 +263,7 @@ win32.relative = function(from, to) {
 
 win32._makeLong = function(path) {
   // Note: this will *probably* throw somewhere.
-  if (!util.isString(path))
+  if (typeof path !== 'string')
     return path;
 
   if (!path) {
@@ -323,7 +321,7 @@ win32.extname = function(path) {
 
 
 win32.format = function(pathObject) {
-  if (!util.isObject(pathObject)) {
+  if (pathObject === null || typeof pathObject !== 'object') {
     throw new TypeError(
         "Parameter 'pathObject' must be an object, not " + typeof pathObject
     );
@@ -331,7 +329,7 @@ win32.format = function(pathObject) {
 
   var root = pathObject.root || '';
 
-  if (!util.isString(root)) {
+  if (typeof root !== 'string') {
     throw new TypeError(
         "'pathObject.root' must be a string or undefined, not " +
         typeof pathObject.root
@@ -353,7 +351,7 @@ win32.format = function(pathObject) {
 
 
 win32.parse = function(pathString) {
-  if (!util.isString(pathString)) {
+  if (typeof pathString !== 'string') {
     throw new TypeError(
         "Parameter 'pathString' must be a string, not " + typeof pathString
     );
@@ -398,7 +396,7 @@ posix.resolve = function() {
     var path = (i >= 0) ? arguments[i] : process.cwd();
 
     // Skip empty and invalid entries
-    if (!util.isString(path)) {
+    if (typeof path !== 'string') {
       throw new TypeError('Arguments to path.resolve must be strings');
     } else if (!path) {
       continue;
@@ -447,7 +445,7 @@ posix.join = function() {
   var path = '';
   for (var i = 0; i < arguments.length; i++) {
     var segment = arguments[i];
-    if (!util.isString(segment)) {
+    if (typeof segment !== 'string') {
       throw new TypeError('Arguments to path.join must be strings');
     }
     if (segment) {
@@ -546,7 +544,7 @@ posix.extname = function(path) {
 
 
 posix.format = function(pathObject) {
-  if (!util.isObject(pathObject)) {
+  if (pathObject === null || typeof pathObject !== 'object') {
     throw new TypeError(
         "Parameter 'pathObject' must be an object, not " + typeof pathObject
     );
@@ -554,7 +552,7 @@ posix.format = function(pathObject) {
 
   var root = pathObject.root || '';
 
-  if (!util.isString(root)) {
+  if (typeof root !== 'string') {
     throw new TypeError(
         "'pathObject.root' must be a string or undefined, not " +
         typeof pathObject.root
@@ -568,7 +566,7 @@ posix.format = function(pathObject) {
 
 
 posix.parse = function(pathString) {
-  if (!util.isString(pathString)) {
+  if (typeof pathString !== 'string') {
     throw new TypeError(
         "Parameter 'pathString' must be a string, not " + typeof pathString
     );
diff --git a/lib/querystring.js b/lib/querystring.js
index 5270bc6f1ab2ff..b574978bc78230 100644
--- a/lib/querystring.js
+++ b/lib/querystring.js
@@ -3,8 +3,6 @@
 'use strict';
 
 const QueryString = exports;
-const util = require('util');
-
 
 // If obj.hasOwnProperty has been overridden, then calling
 // obj.hasOwnProperty(prop) will break.
@@ -100,11 +98,13 @@ QueryString.escape = function(str) {
 };
 
 var stringifyPrimitive = function(v) {
-  if (util.isString(v))
+  let type = typeof v;
+
+  if (type === 'string')
     return v;
-  if (util.isBoolean(v))
+  if (type === 'boolean')
     return v ? 'true' : 'false';
-  if (util.isNumber(v))
+  if (type === 'number')
     return isFinite(v) ? v : '';
   return '';
 };
@@ -119,7 +119,7 @@ QueryString.stringify = QueryString.encode = function(obj, sep, eq, options) {
     encode = options.encodeURIComponent;
   }
 
-  if (util.isObject(obj)) {
+  if (obj !== null && typeof obj === 'object') {
     var keys = Object.keys(obj);
     var fields = [];
 
@@ -128,7 +128,7 @@ QueryString.stringify = QueryString.encode = function(obj, sep, eq, options) {
       var v = obj[k];
       var ks = encode(stringifyPrimitive(k)) + eq;
 
-      if (util.isArray(v)) {
+      if (Array.isArray(v)) {
         for (var j = 0; j < v.length; j++)
           fields.push(ks + encode(stringifyPrimitive(v[j])));
       } else {
@@ -146,7 +146,7 @@ QueryString.parse = QueryString.decode = function(qs, sep, eq, options) {
   eq = eq || '=';
   var obj = {};
 
-  if (!util.isString(qs) || qs.length === 0) {
+  if (typeof qs !== 'string' || qs.length === 0) {
     return obj;
   }
 
@@ -154,7 +154,7 @@ QueryString.parse = QueryString.decode = function(qs, sep, eq, options) {
   qs = qs.split(sep);
 
   var maxKeys = 1000;
-  if (options && util.isNumber(options.maxKeys)) {
+  if (options && typeof options.maxKeys === 'number') {
     maxKeys = options.maxKeys;
   }
 
@@ -192,7 +192,7 @@ QueryString.parse = QueryString.decode = function(qs, sep, eq, options) {
 
     if (!hasOwnProperty(obj, k)) {
       obj[k] = v;
-    } else if (util.isArray(obj[k])) {
+    } else if (Array.isArray(obj[k])) {
       obj[k].push(v);
     } else {
       obj[k] = [obj[k], v];
diff --git a/lib/readline.js b/lib/readline.js
index 1a159c9a1bdef9..04e3f97aacdc99 100644
--- a/lib/readline.js
+++ b/lib/readline.js
@@ -43,13 +43,13 @@ function Interface(input, output, completer, terminal) {
 
   completer = completer || function() { return []; };
 
-  if (!util.isFunction(completer)) {
+  if (typeof completer !== 'function') {
     throw new TypeError('Argument \'completer\' must be a function');
   }
 
   // backwards compat; check the isTTY prop of the output stream
   //  when `terminal` was not specified
-  if (util.isUndefined(terminal) && !util.isNullOrUndefined(output)) {
+  if (terminal === undefined && !(output === null || output === undefined)) {
     terminal = !!output.isTTY;
   }
 
@@ -72,14 +72,15 @@ function Interface(input, output, completer, terminal) {
   }
 
   function onend() {
-    if (util.isString(self._line_buffer) && self._line_buffer.length > 0) {
+    if (typeof self._line_buffer === 'string' &&
+        self._line_buffer.length > 0) {
       self.emit('line', self._line_buffer);
     }
     self.close();
   }
 
   function ontermend() {
-    if (util.isString(self.line) && self.line.length > 0) {
+    if (typeof self.line === 'string' && self.line.length > 0) {
       self.emit('line', self.line);
     }
     self.close();
@@ -123,13 +124,13 @@ function Interface(input, output, completer, terminal) {
     this.history = [];
     this.historyIndex = -1;
 
-    if (!util.isNullOrUndefined(output))
+    if (output !== null && output !== undefined)
       output.on('resize', onresize);
 
     self.once('close', function() {
       input.removeListener('keypress', onkeypress);
       input.removeListener('end', ontermend);
-      if (!util.isNullOrUndefined(output)) {
+      if (output !== null && output !== undefined) {
         output.removeListener('resize', onresize);
       }
     });
@@ -153,7 +154,7 @@ Interface.prototype.setPrompt = function(prompt) {
 
 
 Interface.prototype._setRawMode = function(mode) {
-  if (util.isFunction(this.input.setRawMode)) {
+  if (typeof this.input.setRawMode === 'function') {
     return this.input.setRawMode(mode);
   }
 };
@@ -171,7 +172,7 @@ Interface.prototype.prompt = function(preserveCursor) {
 
 
 Interface.prototype.question = function(query, cb) {
-  if (util.isFunction(cb)) {
+  if (typeof cb === 'function') {
     if (this._questionCallback) {
       this.prompt();
     } else {
@@ -196,10 +197,10 @@ Interface.prototype._onLine = function(line) {
 };
 
 Interface.prototype._writeToOutput = function _writeToOutput(stringToWrite) {
-  if (!util.isString(stringToWrite))
+  if (typeof stringToWrite !== 'string')
     throw new TypeError('stringToWrite must be a string');
 
-  if (!util.isNullOrUndefined(this.output))
+  if (this.output !== null && this.output !== undefined)
     this.output.write(stringToWrite);
 };
 
@@ -296,7 +297,7 @@ Interface.prototype.write = function(d, key) {
 // \r\n, \n, or \r followed by something other than \n
 const lineEnding = /\r?\n|\r(?!\n)/;
 Interface.prototype._normalWrite = function(b) {
-  if (util.isUndefined(b)) {
+  if (b === undefined) {
     return;
   }
   var string = this._decoder.write(b);
@@ -854,7 +855,7 @@ Interface.prototype._ttyWrite = function(s, key) {
         break;
 
       default:
-        if (util.isBuffer(s))
+        if (s instanceof Buffer)
           s = s.toString('utf-8');
 
         if (s) {
@@ -952,8 +953,8 @@ const escapeCodeReAnywhere = new RegExp([
 ].join('|'));
 
 function emitKeys(stream, s) {
-  if (util.isBuffer(s)) {
-    if (s[0] > 127 && util.isUndefined(s[1])) {
+  if (s instanceof Buffer) {
+    if (s[0] > 127 && s[1] === undefined) {
       s[0] -= 128;
       s = '\x1b' + s.toString(stream.encoding || 'utf-8');
     } else {
@@ -1143,7 +1144,7 @@ function emitKeys(stream, s) {
     }
 
     // Don't emit a key if no name was found
-    if (util.isUndefined(key.name)) {
+    if (key.name === undefined) {
       key = undefined;
     }
 
@@ -1163,16 +1164,16 @@ function emitKeys(stream, s) {
  */
 
 function cursorTo(stream, x, y) {
-  if (util.isNullOrUndefined(stream))
+  if (stream === null || stream === undefined)
     return;
 
-  if (!util.isNumber(x) && !util.isNumber(y))
+  if (typeof x !== 'number' && typeof y !== 'number')
     return;
 
-  if (!util.isNumber(x))
+  if (typeof x !== 'number')
     throw new Error("Can't set cursor row without also setting it's column");
 
-  if (!util.isNumber(y)) {
+  if (typeof y !== 'number') {
     stream.write('\x1b[' + (x + 1) + 'G');
   } else {
     stream.write('\x1b[' + (y + 1) + ';' + (x + 1) + 'H');
@@ -1186,7 +1187,7 @@ exports.cursorTo = cursorTo;
  */
 
 function moveCursor(stream, dx, dy) {
-  if (util.isNullOrUndefined(stream))
+  if (stream === null || stream === undefined)
     return;
 
   if (dx < 0) {
@@ -1212,7 +1213,7 @@ exports.moveCursor = moveCursor;
  */
 
 function clearLine(stream, dir) {
-  if (util.isNullOrUndefined(stream))
+  if (stream === null || stream === undefined)
     return;
 
   if (dir < 0) {
@@ -1234,7 +1235,7 @@ exports.clearLine = clearLine;
  */
 
 function clearScreenDown(stream) {
-  if (util.isNullOrUndefined(stream))
+  if (stream === null || stream === undefined)
     return;
 
   stream.write('\x1b[0J');
diff --git a/lib/repl.js b/lib/repl.js
index 64d99b00010966..0a63f325c67098 100644
--- a/lib/repl.js
+++ b/lib/repl.js
@@ -63,7 +63,7 @@ function REPLServer(prompt, stream, eval_, useGlobal, ignoreUndefined) {
   }
 
   var options, input, output, dom;
-  if (util.isObject(prompt)) {
+  if (prompt !== null && typeof prompt === 'object') {
     // an options object was given
     options = prompt;
     stream = options.stream || options.socket;
@@ -74,7 +74,7 @@ function REPLServer(prompt, stream, eval_, useGlobal, ignoreUndefined) {
     ignoreUndefined = options.ignoreUndefined;
     prompt = options.prompt;
     dom = options.domain;
-  } else if (!util.isString(prompt)) {
+  } else if (typeof prompt !== 'string') {
     throw new Error('An options Object, or a prompt String are required');
   } else {
     options = {};
@@ -176,7 +176,7 @@ function REPLServer(prompt, stream, eval_, useGlobal, ignoreUndefined) {
     options.terminal
   ]);
 
-  self.setPrompt(!util.isUndefined(prompt) ? prompt : '> ');
+  self.setPrompt(prompt !== undefined ? prompt : '> ');
 
   this.commands = {};
   defineDefaultCommands(this);
@@ -184,7 +184,7 @@ function REPLServer(prompt, stream, eval_, useGlobal, ignoreUndefined) {
   // figure out which "writer" function to use
   self.writer = options.writer || exports.writer;
 
-  if (util.isUndefined(options.useColors)) {
+  if (options.useColors === undefined) {
     options.useColors = self.terminal;
   }
   self.useColors = !!options.useColors;
@@ -303,7 +303,7 @@ function REPLServer(prompt, stream, eval_, useGlobal, ignoreUndefined) {
       self.bufferedCommand = '';
 
       // If we got any output - print it (if no error)
-      if (!e && (!self.ignoreUndefined || !util.isUndefined(ret))) {
+      if (!e && (!self.ignoreUndefined || ret !== undefined)) {
         self.context._ = ret;
         self.outputStream.write(self.writer(ret) + '\n');
       }
@@ -431,7 +431,7 @@ const simpleExpressionRE =
 // getter code.
 REPLServer.prototype.complete = function(line, callback) {
   // There may be local variables to evaluate, try a nested REPL
-  if (!util.isUndefined(this.bufferedCommand) && this.bufferedCommand.length) {
+  if (this.bufferedCommand !== undefined && this.bufferedCommand.length) {
     // Get a new array of inputed lines
     var tmp = this.lines.slice();
     // Kill off all function declarations to push all local variables into
@@ -573,7 +573,7 @@ REPLServer.prototype.complete = function(line, callback) {
           this.eval('.scope', this.context, 'repl', function(err, globals) {
             if (err || !globals) {
               addStandardGlobals(completionGroups, filter);
-            } else if (util.isArray(globals[0])) {
+            } else if (Array.isArray(globals[0])) {
               // Add grouped globals
               globals.forEach(function(group) {
                 completionGroups.push(group);
@@ -590,19 +590,19 @@ REPLServer.prototype.complete = function(line, callback) {
           // if (e) console.log(e);
 
           if (obj != null) {
-            if (util.isObject(obj) || util.isFunction(obj)) {
+            if (typeof obj === 'object' || typeof obj === 'function') {
               memberGroups.push(Object.getOwnPropertyNames(obj));
             }
             // works for non-objects
             try {
               var sentinel = 5;
               var p;
-              if (util.isObject(obj) || util.isFunction(obj)) {
+              if (typeof obj === 'object' || typeof obj === 'function') {
                 p = Object.getPrototypeOf(obj);
               } else {
                 p = obj.constructor ? obj.constructor.prototype : null;
               }
-              while (!util.isNull(p)) {
+              while (p !== null) {
                 memberGroups.push(Object.getOwnPropertyNames(p));
                 p = Object.getPrototypeOf(p);
                 // Circular refs possible? Let's guard against that.
@@ -701,9 +701,9 @@ REPLServer.prototype.parseREPLKeyword = function(keyword, rest) {
 
 
 REPLServer.prototype.defineCommand = function(keyword, cmd) {
-  if (util.isFunction(cmd)) {
+  if (typeof cmd === 'function') {
     cmd = {action: cmd};
-  } else if (!util.isFunction(cmd.action)) {
+  } else if (typeof cmd.action !== 'function') {
     throw new Error('bad argument, action must be a function');
   }
   this.commands[keyword] = cmd;
diff --git a/lib/smalloc.js b/lib/smalloc.js
index beccba71a4a536..b6eb21fbc3298e 100644
--- a/lib/smalloc.js
+++ b/lib/smalloc.js
@@ -40,10 +40,10 @@ Object.defineProperty(exports, 'Types', {
 function alloc(n, obj, type) {
   n = n >>> 0;
 
-  if (util.isUndefined(obj))
+  if (obj === undefined)
     obj = {};
 
-  if (util.isNumber(obj)) {
+  if (typeof obj === 'number') {
     type = obj >>> 0;
     obj = {};
   } else if (util.isPrimitive(obj)) {
@@ -53,7 +53,7 @@ function alloc(n, obj, type) {
   // 1 == v8::kExternalUint8Array, 9 == v8::kExternalUint8ClampedArray
   if (type < 1 || type > 9)
     throw new TypeError('unknown external array type: ' + type);
-  if (util.isArray(obj))
+  if (Array.isArray(obj))
     throw new TypeError('Arrays are not supported');
   if (n > kMaxLength)
     throw new RangeError('n > kMaxLength');
@@ -65,7 +65,7 @@ function alloc(n, obj, type) {
 function dispose(obj) {
   if (util.isPrimitive(obj))
     throw new TypeError('obj must be an Object');
-  if (util.isBuffer(obj))
+  if (obj instanceof Buffer)
     throw new TypeError('obj cannot be a Buffer');
   if (smalloc.isTypedArray(obj))
     throw new TypeError('obj cannot be a typed array');
diff --git a/lib/stream.js b/lib/stream.js
index bbd6bc5aae2f3e..dd381d97374dd5 100644
--- a/lib/stream.js
+++ b/lib/stream.js
@@ -65,7 +65,7 @@ Stream.prototype.pipe = function(dest, options) {
     if (didOnEnd) return;
     didOnEnd = true;
 
-    if (util.isFunction(dest.destroy)) dest.destroy();
+    if (typeof dest.destroy === 'function') dest.destroy();
   }
 
   // don't leave dangling pipes when there are errors.
diff --git a/lib/tls.js b/lib/tls.js
index d52adbabc83104..0e13516add634e 100644
--- a/lib/tls.js
+++ b/lib/tls.js
@@ -36,7 +36,7 @@ exports.getCiphers = function() {
 // ("\x06spdy/2\x08http/1.1\x08http/1.0")
 exports.convertNPNProtocols = function convertNPNProtocols(NPNProtocols, out) {
   // If NPNProtocols is Array - translate it into buffer
-  if (util.isArray(NPNProtocols)) {
+  if (Array.isArray(NPNProtocols)) {
     var buff = new Buffer(NPNProtocols.reduce(function(p, c) {
       return p + 1 + Buffer.byteLength(c);
     }, 0));
@@ -53,7 +53,7 @@ exports.convertNPNProtocols = function convertNPNProtocols(NPNProtocols, out) {
   }
 
   // If it's already a Buffer - store it
-  if (util.isBuffer(NPNProtocols)) {
+  if (NPNProtocols instanceof Buffer) {
     out.NPNProtocols = NPNProtocols;
   }
 };
@@ -160,7 +160,7 @@ exports.checkServerIdentity = function checkServerIdentity(host, cert) {
     // RFC6125
     if (matchCN) {
       var commonNames = cert.subject.CN;
-      if (util.isArray(commonNames)) {
+      if (Array.isArray(commonNames)) {
         for (var i = 0, k = commonNames.length; i < k; ++i) {
           dnsNames.push(regexpify(commonNames[i], true));
         }
@@ -208,7 +208,7 @@ exports.parseCertString = function parseCertString(s) {
       var key = parts[i].slice(0, sepIndex);
       var value = parts[i].slice(sepIndex + 1);
       if (key in out) {
-        if (!util.isArray(out[key])) {
+        if (!Array.isArray(out[key])) {
           out[key] = [out[key]];
         }
         out[key].push(value);
diff --git a/lib/tty.js b/lib/tty.js
index 84731a5e75c623..1241d52535cfeb 100644
--- a/lib/tty.js
+++ b/lib/tty.js
@@ -1,11 +1,10 @@
 'use strict';
 
-const inherits = require('util').inherits;
+const util = require('util');
 const net = require('net');
 const TTY = process.binding('tty_wrap').TTY;
 const isTTY = process.binding('tty_wrap').isTTY;
-const util = require('util');
-
+const inherits = util.inherits;
 const errnoException = util._errnoException;
 
 
diff --git a/lib/url.js b/lib/url.js
index 50126ae9bfc4d0..65012cd7c45bf9 100644
--- a/lib/url.js
+++ b/lib/url.js
@@ -1,7 +1,6 @@
 'use strict';
 
 const punycode = require('punycode');
-const util = require('util');
 
 exports.parse = urlParse;
 exports.resolve = urlResolve;
@@ -79,7 +78,7 @@ const slashedProtocol = {
 const querystring = require('querystring');
 
 function urlParse(url, parseQueryString, slashesDenoteHost) {
-  if (url && util.isObject(url) && url instanceof Url) return url;
+  if (url instanceof Url) return url;
 
   var u = new Url;
   u.parse(url, parseQueryString, slashesDenoteHost);
@@ -87,7 +86,7 @@ function urlParse(url, parseQueryString, slashesDenoteHost) {
 }
 
 Url.prototype.parse = function(url, parseQueryString, slashesDenoteHost) {
-  if (!util.isString(url)) {
+  if (typeof url !== 'string') {
     throw new TypeError("Parameter 'url' must be a string, not " + typeof url);
   }
 
@@ -353,7 +352,7 @@ function urlFormat(obj) {
   // If it's an obj, this is a no-op.
   // this way, you can call url_format() on strings
   // to clean up potentially wonky urls.
-  if (util.isString(obj)) obj = urlParse(obj);
+  if (typeof obj === 'string') obj = urlParse(obj);
   if (!(obj instanceof Url)) return Url.prototype.format.call(obj);
   return obj.format();
 }
@@ -383,8 +382,8 @@ Url.prototype.format = function() {
     }
   }
 
-  if (this.query &&
-      util.isObject(this.query) &&
+  if (this.query !== null &&
+      typeof this.query === 'object' &&
       Object.keys(this.query).length) {
     query = querystring.stringify(this.query);
   }
@@ -428,7 +427,7 @@ function urlResolveObject(source, relative) {
 }
 
 Url.prototype.resolveObject = function(relative) {
-  if (util.isString(relative)) {
+  if (typeof relative === 'string') {
     var rel = new Url();
     rel.parse(relative, false, true);
     relative = rel;
@@ -574,7 +573,7 @@ Url.prototype.resolveObject = function(relative) {
     srcPath = srcPath.concat(relPath);
     result.search = relative.search;
     result.query = relative.query;
-  } else if (!util.isNullOrUndefined(relative.search)) {
+  } else if (relative.search !== null && relative.search !== undefined) {
     // just pull out the search.
     // like href='?foo'.
     // Put this after the other two cases because it simplifies the booleans
@@ -593,7 +592,7 @@ Url.prototype.resolveObject = function(relative) {
     result.search = relative.search;
     result.query = relative.query;
     //to support http.request
-    if (!util.isNull(result.pathname) || !util.isNull(result.search)) {
+    if (result.pathname !== null || result.search !== null) {
       result.path = (result.pathname ? result.pathname : '') +
                     (result.search ? result.search : '');
     }
@@ -687,7 +686,7 @@ Url.prototype.resolveObject = function(relative) {
   }
 
   //to support request.http
-  if (!util.isNull(result.pathname) || !util.isNull(result.search)) {
+  if (result.pathname !== null || result.search !== null) {
     result.path = (result.pathname ? result.pathname : '') +
                   (result.search ? result.search : '');
   }
diff --git a/lib/util.js b/lib/util.js
index 7c3e03c24e09fd..f84f528ce3b78e 100644
--- a/lib/util.js
+++ b/lib/util.js
@@ -2,7 +2,7 @@
 
 const formatRegExp = /%[sdj%]/g;
 exports.format = function(f) {
-  if (!isString(f)) {
+  if (typeof f !== 'string') {
     var objects = [];
     for (var i = 0; i < arguments.length; i++) {
       objects.push(inspect(arguments[i]));
@@ -30,7 +30,7 @@ exports.format = function(f) {
     }
   });
   for (var x = args[i]; i < len; x = args[++i]) {
-    if (isNull(x) || !isObject(x)) {
+    if (x === null || typeof x !== 'object') {
       str += ' ' + x;
     } else {
       str += ' ' + inspect(x);
@@ -45,7 +45,7 @@ exports.format = function(f) {
 // If --no-deprecation is set, then it is a no-op.
 exports.deprecate = function(fn, msg) {
   // Allow for deprecating things in the process of starting up.
-  if (isUndefined(global.process)) {
+  if (global.process === undefined) {
     return function() {
       return exports.deprecate(fn, msg).apply(this, arguments);
     };
@@ -77,7 +77,7 @@ exports.deprecate = function(fn, msg) {
 var debugs = {};
 var debugEnviron;
 exports.debuglog = function(set) {
-  if (isUndefined(debugEnviron))
+  if (debugEnviron === undefined)
     debugEnviron = process.env.NODE_DEBUG || '';
   set = set.toUpperCase();
   if (!debugs[set]) {
@@ -112,7 +112,7 @@ function inspect(obj, opts) {
   // legacy...
   if (arguments.length >= 3) ctx.depth = arguments[2];
   if (arguments.length >= 4) ctx.colors = arguments[3];
-  if (isBoolean(opts)) {
+  if (typeof opts === 'boolean') {
     // legacy...
     ctx.showHidden = opts;
   } else if (opts) {
@@ -120,10 +120,10 @@ function inspect(obj, opts) {
     exports._extend(ctx, opts);
   }
   // set default options
-  if (isUndefined(ctx.showHidden)) ctx.showHidden = false;
-  if (isUndefined(ctx.depth)) ctx.depth = 2;
-  if (isUndefined(ctx.colors)) ctx.colors = false;
-  if (isUndefined(ctx.customInspect)) ctx.customInspect = true;
+  if (ctx.showHidden === undefined) ctx.showHidden = false;
+  if (ctx.depth === undefined) ctx.depth = 2;
+  if (ctx.colors === undefined) ctx.colors = false;
+  if (ctx.customInspect === undefined) ctx.customInspect = true;
   if (ctx.colors) ctx.stylize = stylizeWithColor;
   return formatValue(ctx, obj, ctx.depth);
 }
@@ -195,13 +195,13 @@ function formatValue(ctx, value, recurseTimes) {
   // Check that value is an object with an inspect function on it
   if (ctx.customInspect &&
       value &&
-      isFunction(value.inspect) &&
+      typeof value.inspect === 'function' &&
       // Filter out the util module, it's inspect function is special
       value.inspect !== exports.inspect &&
       // Also filter out any prototype objects using the circular check.
       !(value.constructor && value.constructor.prototype === value)) {
     var ret = value.inspect(recurseTimes, ctx);
-    if (!isString(ret)) {
+    if (typeof ret !== 'string') {
       ret = formatValue(ctx, ret, recurseTimes);
     }
     return ret;
@@ -236,7 +236,7 @@ function formatValue(ctx, value, recurseTimes) {
     // ignore...
   }
 
-  if (isString(raw)) {
+  if (typeof raw === 'string') {
     // for boxed Strings, we have to remove the 0-n indexed entries,
     // since they just noisey up the output and are redundant
     keys = keys.filter(function(key) {
@@ -246,7 +246,7 @@ function formatValue(ctx, value, recurseTimes) {
 
   // Some type of object without properties can be shortcutted.
   if (keys.length === 0) {
-    if (isFunction(value)) {
+    if (typeof value === 'function') {
       var name = value.name ? ': ' + value.name : '';
       return ctx.stylize('[Function' + name + ']', 'special');
     }
@@ -260,15 +260,15 @@ function formatValue(ctx, value, recurseTimes) {
       return formatError(value);
     }
     // now check the `raw` value to handle boxed primitives
-    if (isString(raw)) {
+    if (typeof raw === 'string') {
       formatted = formatPrimitiveNoColor(ctx, raw);
       return ctx.stylize('[String: ' + formatted + ']', 'string');
     }
-    if (isNumber(raw)) {
+    if (typeof raw === 'number') {
       formatted = formatPrimitiveNoColor(ctx, raw);
       return ctx.stylize('[Number: ' + formatted + ']', 'number');
     }
-    if (isBoolean(raw)) {
+    if (typeof raw === 'boolean') {
       formatted = formatPrimitiveNoColor(ctx, raw);
       return ctx.stylize('[Boolean: ' + formatted + ']', 'boolean');
     }
@@ -277,13 +277,13 @@ function formatValue(ctx, value, recurseTimes) {
   var base = '', array = false, braces = ['{', '}'];
 
   // Make Array say that they are Array
-  if (isArray(value)) {
+  if (Array.isArray(value)) {
     array = true;
     braces = ['[', ']'];
   }
 
   // Make functions say that they are functions
-  if (isFunction(value)) {
+  if (typeof value === 'function') {
     var n = value.name ? ': ' + value.name : '';
     base = ' [Function' + n + ']';
   }
@@ -304,19 +304,19 @@ function formatValue(ctx, value, recurseTimes) {
   }
 
   // Make boxed primitive Strings look like such
-  if (isString(raw)) {
+  if (typeof raw === 'string') {
     formatted = formatPrimitiveNoColor(ctx, raw);
     base = ' ' + '[String: ' + formatted + ']';
   }
 
   // Make boxed primitive Numbers look like such
-  if (isNumber(raw)) {
+  if (typeof raw === 'number') {
     formatted = formatPrimitiveNoColor(ctx, raw);
     base = ' ' + '[Number: ' + formatted + ']';
   }
 
   // Make boxed primitive Booleans look like such
-  if (isBoolean(raw)) {
+  if (typeof raw === 'boolean') {
     formatted = formatPrimitiveNoColor(ctx, raw);
     base = ' ' + '[Boolean: ' + formatted + ']';
   }
@@ -351,28 +351,32 @@ function formatValue(ctx, value, recurseTimes) {
 
 
 function formatPrimitive(ctx, value) {
-  if (isUndefined(value))
+  if (value === undefined)
     return ctx.stylize('undefined', 'undefined');
-  if (isString(value)) {
+
+  // For some reason typeof null is "object", so special case here.
+  if (value === null)
+    return ctx.stylize('null', 'null');
+
+  var type = typeof value;
+
+  if (type === 'string') {
     var simple = '\'' + JSON.stringify(value).replace(/^"|"$/g, '')
         .replace(/'/g, "\\'")
         .replace(/\\"/g, '"') + '\'';
     return ctx.stylize(simple, 'string');
   }
-  if (isNumber(value)) {
+  if (type === 'number') {
     // Format -0 as '-0'. Strict equality won't distinguish 0 from -0,
     // so instead we use the fact that 1 / -0 < 0 whereas 1 / 0 > 0 .
     if (value === 0 && 1 / value < 0)
       return ctx.stylize('-0', 'number');
     return ctx.stylize('' + value, 'number');
   }
-  if (isBoolean(value))
+  if (type === 'boolean')
     return ctx.stylize('' + value, 'boolean');
-  // For some reason typeof null is "object", so special case here.
-  if (isNull(value))
-    return ctx.stylize('null', 'null');
   // es6 symbol primitive
-  if (isSymbol(value))
+  if (type === 'symbol')
     return ctx.stylize(value.toString(), 'symbol');
 }
 
@@ -402,7 +406,7 @@ function formatArray(ctx, value, recurseTimes, visibleKeys, keys) {
     }
   }
   keys.forEach(function(key) {
-    if (isSymbol(key) || !key.match(/^\d+$/)) {
+    if (typeof key === 'symbol' || !key.match(/^\d+$/)) {
       output.push(formatProperty(ctx, value, recurseTimes, visibleKeys,
           key, true));
     }
@@ -426,7 +430,7 @@ function formatProperty(ctx, value, recurseTimes, visibleKeys, key, array) {
     }
   }
   if (!hasOwnProperty(visibleKeys, key)) {
-    if (isSymbol(key)) {
+    if (typeof key === 'symbol') {
       name = '[' + ctx.stylize(key.toString(), 'symbol') + ']';
     } else {
       name = '[' + key + ']';
@@ -434,7 +438,7 @@ function formatProperty(ctx, value, recurseTimes, visibleKeys, key, array) {
   }
   if (!str) {
     if (ctx.seen.indexOf(desc.value) < 0) {
-      if (isNull(recurseTimes)) {
+      if (recurseTimes === null) {
         str = formatValue(ctx, desc.value, null);
       } else {
         str = formatValue(ctx, desc.value, recurseTimes - 1);
@@ -454,7 +458,7 @@ function formatProperty(ctx, value, recurseTimes, visibleKeys, key, array) {
       str = ctx.stylize('[Circular]', 'special');
     }
   }
-  if (isUndefined(name)) {
+  if (name === undefined) {
     if (array && key.match(/^\d+$/)) {
       return str;
     }
@@ -508,7 +512,7 @@ function isNull(arg) {
 exports.isNull = isNull;
 
 function isNullOrUndefined(arg) {
-  return arg == null;
+  return arg === null || arg === undefined;
 }
 exports.isNullOrUndefined = isNullOrUndefined;
 
@@ -528,27 +532,29 @@ function isSymbol(arg) {
 exports.isSymbol = isSymbol;
 
 function isUndefined(arg) {
-  return arg === void 0;
+  return arg === undefined;
 }
 exports.isUndefined = isUndefined;
 
 function isRegExp(re) {
-  return isObject(re) && objectToString(re) === '[object RegExp]';
+  return re !== null && typeof re === 'object' &&
+         objectToString(re) === '[object RegExp]';
 }
 exports.isRegExp = isRegExp;
 
 function isObject(arg) {
-  return typeof arg === 'object' && arg !== null;
+  return arg !== null && typeof arg === 'object';
 }
 exports.isObject = isObject;
 
 function isDate(d) {
-  return isObject(d) && objectToString(d) === '[object Date]';
+  return d !== null && typeof d === 'object' &&
+         objectToString(d) === '[object Date]';
 }
 exports.isDate = isDate;
 
 function isError(e) {
-  return isObject(e) &&
+  return e !== null && typeof e === 'object' &&
       (objectToString(e) === '[object Error]' || e instanceof Error);
 }
 exports.isError = isError;
@@ -629,7 +635,7 @@ exports.inherits = function(ctor, superCtor) {
 
 exports._extend = function(origin, add) {
   // Don't do anything if add isn't an object
-  if (!add || !isObject(add)) return origin;
+  if (add === null || typeof add !== 'object') return origin;
 
   var keys = Object.keys(add);
   var i = keys.length;
@@ -724,7 +730,7 @@ exports.pump = exports.deprecate(function(readStream, writeStream, callback) {
 
 var uv;
 exports._errnoException = function(err, syscall, original) {
-  if (isUndefined(uv)) uv = process.binding('uv');
+  if (uv === undefined) uv = process.binding('uv');
   var errname = uv.errname(err);
   var message = syscall + ' ' + errname;
   if (original)
diff --git a/lib/vm.js b/lib/vm.js
index 8a190fa9ed7c43..b4a2b9999091d2 100644
--- a/lib/vm.js
+++ b/lib/vm.js
@@ -2,7 +2,6 @@
 
 const binding = process.binding('contextify');
 const Script = binding.ContextifyScript;
-const util = require('util');
 
 // The binding provides a few useful primitives:
 // - ContextifyScript(code, { filename = "evalmachine.anonymous",
@@ -26,7 +25,7 @@ exports.createScript = function(code, options) {
 };
 
 exports.createContext = function(sandbox) {
-  if (util.isUndefined(sandbox)) {
+  if (sandbox === undefined) {
     sandbox = {};
   } else if (binding.isContext(sandbox)) {
     return sandbox;
diff --git a/lib/zlib.js b/lib/zlib.js
index 53e8eb66fdad25..14ccf8f9597731 100644
--- a/lib/zlib.js
+++ b/lib/zlib.js
@@ -92,7 +92,7 @@ exports.createUnzip = function(o) {
 // Convenience methods.
 // compress/decompress a string or buffer in one step.
 exports.deflate = function(buffer, opts, callback) {
-  if (util.isFunction(opts)) {
+  if (typeof opts === 'function') {
     callback = opts;
     opts = {};
   }
@@ -104,7 +104,7 @@ exports.deflateSync = function(buffer, opts) {
 };
 
 exports.gzip = function(buffer, opts, callback) {
-  if (util.isFunction(opts)) {
+  if (typeof opts === 'function') {
     callback = opts;
     opts = {};
   }
@@ -116,7 +116,7 @@ exports.gzipSync = function(buffer, opts) {
 };
 
 exports.deflateRaw = function(buffer, opts, callback) {
-  if (util.isFunction(opts)) {
+  if (typeof opts === 'function') {
     callback = opts;
     opts = {};
   }
@@ -128,7 +128,7 @@ exports.deflateRawSync = function(buffer, opts) {
 };
 
 exports.unzip = function(buffer, opts, callback) {
-  if (util.isFunction(opts)) {
+  if (typeof opts === 'function') {
     callback = opts;
     opts = {};
   }
@@ -140,7 +140,7 @@ exports.unzipSync = function(buffer, opts) {
 };
 
 exports.inflate = function(buffer, opts, callback) {
-  if (util.isFunction(opts)) {
+  if (typeof opts === 'function') {
     callback = opts;
     opts = {};
   }
@@ -152,7 +152,7 @@ exports.inflateSync = function(buffer, opts) {
 };
 
 exports.gunzip = function(buffer, opts, callback) {
-  if (util.isFunction(opts)) {
+  if (typeof opts === 'function') {
     callback = opts;
     opts = {};
   }
@@ -164,7 +164,7 @@ exports.gunzipSync = function(buffer, opts) {
 };
 
 exports.inflateRaw = function(buffer, opts, callback) {
-  if (util.isFunction(opts)) {
+  if (typeof opts === 'function') {
     callback = opts;
     opts = {};
   }
@@ -209,9 +209,9 @@ function zlibBuffer(engine, buffer, callback) {
 }
 
 function zlibBufferSync(engine, buffer) {
-  if (util.isString(buffer))
+  if (typeof buffer === 'string')
     buffer = new Buffer(buffer);
-  if (!util.isBuffer(buffer))
+  if (!(buffer instanceof Buffer))
     throw new TypeError('Not a string or buffer');
 
   var flushFlag = binding.Z_FINISH;
@@ -327,7 +327,7 @@ function Zlib(opts, mode) {
   }
 
   if (opts.dictionary) {
-    if (!util.isBuffer(opts.dictionary)) {
+    if (!(opts.dictionary instanceof Buffer)) {
       throw new Error('Invalid dictionary: it should be a Buffer instance');
     }
   }
@@ -349,10 +349,10 @@ function Zlib(opts, mode) {
   };
 
   var level = exports.Z_DEFAULT_COMPRESSION;
-  if (util.isNumber(opts.level)) level = opts.level;
+  if (typeof opts.level === 'number') level = opts.level;
 
   var strategy = exports.Z_DEFAULT_STRATEGY;
-  if (util.isNumber(opts.strategy)) strategy = opts.strategy;
+  if (typeof opts.strategy === 'number') strategy = opts.strategy;
 
   this._handle.init(opts.windowBits || exports.Z_DEFAULT_WINDOWBITS,
                     level,
@@ -414,7 +414,7 @@ Zlib.prototype._flush = function(callback) {
 Zlib.prototype.flush = function(kind, callback) {
   var ws = this._writableState;
 
-  if (util.isFunction(kind) || (util.isUndefined(kind) && !callback)) {
+  if (typeof kind === 'function' || (kind === undefined && !callback)) {
     callback = kind;
     kind = binding.Z_FULL_FLUSH;
   }
@@ -459,7 +459,7 @@ Zlib.prototype._transform = function(chunk, encoding, cb) {
   var ending = ws.ending || ws.ended;
   var last = ending && (!chunk || ws.length === chunk.length);
 
-  if (!util.isNull(chunk) && !util.isBuffer(chunk))
+  if (chunk !== null && !(chunk instanceof Buffer))
     return cb(new Error('invalid input'));
 
   if (this._closed)
@@ -490,7 +490,7 @@ Zlib.prototype._processChunk = function(chunk, flushFlag, cb) {
 
   var self = this;
 
-  var async = util.isFunction(cb);
+  var async = typeof cb === 'function';
 
   if (!async) {
     var buffers = [];