Line numbers: Added inheritance for the line-numbers class (#1799)

This adds inheritance for the `line-numbers` class for the Line numbers plugin similar to the inheritance of `language-xxxx` classes.

plugins/line-numbers/index.html

@@ -27,19 +27,20 @@ <h2>Line Numbers</h2>
   <h1>How to use</h1>
 
   <p>Obviously, this is supposed to work only for code blocks (<code>&lt;pre>&lt;code></code>) and not for inline code.</p>
-  <p>Add the <strong>line-numbers</strong> class to your desired <code>&lt;pre></code>, and the line-numbers plugin will take care of the rest.</p>
+  <p>Add the <code>line-numbers</code> class to your desired <code>&lt;pre></code> or any of its ancestors, and the line-numbers plugin will take care of the rest. To given all code blocks line numbers, add the <code>line-numbers</code> class to the <code>&lt;body></code> of the page.</p>
   <p>Optional: You can specify the <code>data-start</code> (Number) attribute on the <code>&lt;pre></code> element. It will shift the line counter.</p>
   <p>Optional: To support multiline line numbers using soft wrap, apply the CSS <code>white-space: pre-line;</code> or <code>white-space: pre-wrap;</code> to your desired <code>&lt;pre></code>.</p>
 </section>
 
-<section>
+<section class="line-numbers">
   <h1>Examples</h1>
 
   <h2>JavaScript</h2>
   <pre class="line-numbers" data-src="plugins/line-numbers/prism-line-numbers.js"></pre>
 
   <h2>CSS</h2>
-  <pre class="line-numbers" data-src="plugins/line-numbers/prism-line-numbers.css"></pre>
+  <p>Please note that this <code class="language-markup">&lt;pre></code> does not have the <code>line-numbers</code> class but its parent does.</p>
+  <pre data-src="plugins/line-numbers/prism-line-numbers.css"></pre>
 
   <h2>HTML</h2>
   <p>Please note the <code>data-start="-5"</code> in the code below.</p>

plugins/line-numbers/prism-line-numbers.js

@@ -71,37 +71,46 @@
 			return;
 		}
 
+		var code = env.element;
+		var pre = code.parentNode;
+
 		// works only for <code> wrapped inside <pre> (not inline)
-		var pre = env.element.parentNode;
-		var clsReg = /(?:^|\s)line-numbers(?:\s|$)/;
-		if (
-			!pre || !/pre/i.test(pre.nodeName) ||
-			// Abort only if nor the <pre> nor the <code> have the class
-			(!clsReg.test(pre.className) && !clsReg.test(env.element.className))
-		) {
+		if (!pre || !/pre/i.test(pre.nodeName)) {
 			return;
 		}
 
-		if (env.element.querySelector('.line-numbers-rows')) {
-			// Abort if line numbers already exists
+		// Abort if line numbers already exists
+		if (code.querySelector('.line-numbers-rows')) {
 			return;
 		}
 
-		if (clsReg.test(env.element.className)) {
-			// Remove the class 'line-numbers' from the <code>
-			env.element.className = env.element.className.replace(clsReg, ' ');
+		var addLineNumbers = false;
+		var lineNumbersRegex = /(?:^|\s)line-numbers(?:\s|$)/;
+
+		for (var element = code; element; element = element.parentNode) {
+			if (lineNumbersRegex.test(element.className)) {
+				addLineNumbers = true;
+				break;
+			}
 		}
-		if (!clsReg.test(pre.className)) {
-			// Add the class 'line-numbers' to the <pre>
+
+		// only add line numbers if <code> or one of its ancestors has the `line-numbers` class
+		if (!addLineNumbers) {
+			return;
+		}
+
+		// Remove the class 'line-numbers' from the <code>
+		code.className = code.className.replace(lineNumbersRegex, ' ');
+		// Add the class 'line-numbers' to the <pre>
+		if (!lineNumbersRegex.test(pre.className)) {
 			pre.className += ' line-numbers';
 		}
 
 		var match = env.code.match(NEW_LINE_EXP);
 		var linesNum = match ? match.length + 1 : 1;
 		var lineNumbersWrapper;
 
-		var lines = new Array(linesNum + 1);
-		lines = lines.join('<span></span>');
+		var lines = new Array(linesNum + 1).join('<span></span>');
 
 		lineNumbersWrapper = document.createElement('span');
 		lineNumbersWrapper.setAttribute('aria-hidden', 'true');