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copied!<p>ECMAScript <em>does</em> have <a href="http://en.wikipedia.org/wiki/Associative_array" rel="nofollow">Associated Arrays</a><sup>1</sup> - Objects (and by extension, Arrays) are an example</p> <p><em>However</em>, <strong><a href="http://es5.github.com/#x15.4" rel="nofollow">some properties of Arrays are treated specially</a></strong>:</p> <blockquote> <p><em>Array objects give special treatment to a certain class of property names.</em> A property name <code>P</code> (in the form of a String value) is an array index if and only if <code>ToString(ToUint32(P))</code> is equal to <code>P</code> ..</p> <p>.. <em>Specifically, whenever a property is added whose name is an array index, the length property is changed, if necessary,</em> to be one more than the numeric value of that array index ..</p> </blockquote> <p>Thus, given <code>arr = []</code>, the expressions <code>arr["1"]</code> and <code>arr[1]</code> refer to the <em>same property</em> name. Since <code>P</code> (the property name) is "1" and <code>length</code> is 0 from above, then assignment to such property will set <code>arr.length</code> to <code>ToUint32(P)+1</code>, or 2.</p> <p>It is <em>not</em> possible to change this behavior. If you wish to <em>not</em> have a special <code>length</code> property, then use a "normal" Object instead of an Array. However, many of the <code>Array.prototype</code> functions <em>can</em> be used with arbitrary objects (with some implementation quirks aside) that have a <code>length</code> property and an Object <em>can</em> be created such that it uses <code>Array.prototype</code> as its own prototype.</p> <p>All that being said, the post <em>does not say what the real issue is</em>. Instead of supposing that it must be done in that particular manner, consider explaining what the intent is: e.g. why a["10"]? And what is wrong if there are "11 items" if the object will be used in a List?</p> <hr> <p><sup>1</sup> Please read the article before debating this statement: the term "Array" in the name does not imply an ordered sequence nor does it preclude an <em>additional</em> notion of a Length or the use of Hashing, etc. If you are going by a <em>different</em> definition, make sure to specify what it is and what the desired behavior is for a given operation.</p>

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