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spec.html
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<!doctype html>
<meta charset="utf8">
<script src="ecmarkup.js"></script>
<link rel="stylesheet" href="ecmarkup.css">
<title>Relative Indexing Method</title>
<pre class=metadata>
title: Relative Indexing Method
status: proposal
stage: 4
location: https://github.com/tabatkins/proposal-relative-indexing-method
copyright: false
contributors: Tab Atkins, Shu-yu Guo
</pre>
<emu-intro id="intro">
<h1>Relative Indexing Method</h1>
<p>We provide a new method, `at()`, that is on the prototype of the built-in indexable objects: Array, String, and TypedArrays objects. The method supports relative indexing from the end when passed a negative index.</p>
</emu-intro>
<emu-clause id="sec-array-prototype-additions">
<h1>Additions to Properties of the Array Prototype Object</h1>
<emu-clause id="sec-array.prototype.at">
<h1>Array.prototype.at ( _index_ )</h1>
<emu-alg>
1. Let _O_ be ? ToObject(*this* value).
1. Let _len_ be ? LengthOfArrayLike(_O_).
1. Let _relativeIndex_ be ? ToInteger(_index_).
1. If _relativeIndex_ ≥ 0, then
1. Let _k_ be _relativeIndex_.
1. Else,
1. Let _k_ be _len_ + _relativeIndex_.
1. If _k_ < 0 or _k_ ≥ _len_, then return *undefined*.
1. Return ? Get(_O_, ! ToString(_k_)).
</emu-alg>
</emu-clause>
</emu-clause>
<emu-clause id="sec-array.prototype-modifications">
<h1>Modifications to Properties of the Array Prototype Object</h1>
<emu-clause id="sec-array.prototype-@@unscopables-modification">
<h1>Array.prototype [ @@unscopables ]</h1>
<p>The initial value of the @@unscopables data property is an object created by the following steps:</p>
<emu-alg>
1. Let _unscopableList_ be ! OrdinaryObjectCreate(*null*).
1. <ins>Perform ! CreateDataPropertyOrThrow(_unscopableList_, *"at"*, *true*).</ins>
1. Perform ! CreateDataPropertyOrThrow(_unscopableList_, *"copyWithin"*, *true*).
1. Perform ! CreateDataPropertyOrThrow(_unscopableList_, *"entries"*, *true*).
1. Perform ! CreateDataPropertyOrThrow(_unscopableList_, *"fill"*, *true*).
1. Perform ! CreateDataPropertyOrThrow(_unscopableList_, *"find"*, *true*).
1. Perform ! CreateDataPropertyOrThrow(_unscopableList_, *"findIndex"*, *true*).
1. Perform ! CreateDataPropertyOrThrow(_unscopableList_, *"flat"*, *true*).
1. Perform ! CreateDataPropertyOrThrow(_unscopableList_, *"flatMap"*, *true*).
1. Perform ! CreateDataPropertyOrThrow(_unscopableList_, *"includes"*, *true*).
1. Perform ! CreateDataPropertyOrThrow(_unscopableList_, *"keys"*, *true*).
1. Perform ! CreateDataPropertyOrThrow(_unscopableList_, *"values"*, *true*).
1. Return _unscopableList_.
</emu-alg>
</emu-clause>
</emu-clause>
<emu-clause id="sec-string-prototype-additions">
<h1>Additions to Properties of the String Prototype Object</h1>
<emu-clause id="sec-string.prototype.at">
<h1>String.prototype.at ( _index_ )</h1>
<emu-alg>
1. Let _O_ be ? RequireObjectCoercible(*this* value).
1. Let _S_ be ? ToString(_O_).
1. Let _len_ be the length of _S_.
1. Let _relativeIndex_ be ? ToInteger(_index_).
1. If _relativeIndex_ ≥ 0, then
1. Let _k_ be _relativeIndex_.
1. Else,
1. Let _k_ be _len_ + _relativeIndex_.
1. If _k_ < 0 or _k_ ≥ _len_, then return *undefined*.
1. Return the String value consisting of only the code unit at position _k_ in _S_.
</emu-alg>
</emu-clause>
</emu-clause>
<emu-clause id="sec-%typedarray.prototype%-additions">
<h1>Additions to Properties of the %TypedArray.prototype% Object</h1>
<emu-clause id="sec-%typedarray%.prototype.at">
<h1>%TypedArray%.prototype.at ( _index_ )</h1>
<emu-alg>
1. Let _O_ be the *this* value.
1. Perform ? ValidateTypedArray(_O_).
1. Let _len_ be _O_.[[ArrayLength]].
1. Let _relativeIndex_ be ? ToInteger(_index_).
1. If _relativeIndex_ ≥ 0, then
1. Let _k_ be _relativeIndex_.
1. Else,
1. Let _k_ be _len_ + _relativeIndex_.
1. If _k_ < 0 or _k_ ≥ _len_, then return *undefined*.
1. Return ? Get(_O_, ! ToString(_k_)).
</emu-alg>
</emu-clause>
</emu-clause>