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POHow does Javascript's indexOf() resolve references
 

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  1. POHow does Javascript's indexOf() resolve references
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    <p>I was confusing myself a little with a thought experiment and now I'm looking for some advice. Its about ECMAscript references and the <code>Array.prototype.indexOf()</code> method.</p> <p>Lets start easy:</p> <pre><code>var container = [ ]; // more code container.push( 5 ); container.push( 7 ); container.push( 10 ); </code></pre> <hr> <p>So now we pushed some "<em>primitive values</em>" into our ECMAscript array (whether or not that statement is true I'll come back for), at least I imagined it like this so far. A call to</p> <pre><code>container.indexOf( 7 ); </code></pre> <p>will return <code>1</code> as expected. The big question I'm having is, if <code>.indexOf()</code> really compares the primitive value or if in reality a <code>Number()</code> object is created + stored and its <strong>reference</strong> is getting compared. This becomes a little more obvious if we re-write that like so:</p> <pre><code>var a = 5, b = 7, c = 10; var container = [ ]; container.push( a ); container.push( b ); container.push( c ); container.indexOf( b ); </code></pre> <hr> <p>Until this point, one could still easily argue that all <code>.indexOf()</code> needs to do is to compare values, but now lets look at something like this:</p> <pre><code>var a = { name: 'a', value: 5 }, b = { name: 'b', value: 10 }, c = { name: 'c', value: 15 }; var container = [ ]; // more code container.push( a ); container.push( b ); container.push( c ); </code></pre> <p>Here, we filled that container array with <em>object-references</em> and still, <code>.indexOf()</code> works as expected</p> <pre><code>container.indexOf( b ) // === 1 </code></pre> <p>while a call like this</p> <pre><code>container.indexOf({ name: 'b', value: 10 }); </code></pre> <p>obviously returns <code>-1</code> since we are creating a new object and get a new reference. So here it must internally compare references with each other, right?</p> <p>Can some ECMAscript spec genius confirm that or even better link me some material about that ?</p> <p>A side question on this would be if there is any possibly way to access an internally stored object-reference within a <em>lexicalEnvironment</em> respectively <em>Activation Object</em>.</p>
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    1. COIf this works anywhere similar to other languages, then `indexOf` works on object hashes where objects usually have different values and primitive objects usually have a constant hash based on their value (for integers, this is very often just the integer value itself). So two int objects `5` and `5` both have the hash `5` and as such are “the same”.
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    2. COI started writing an answer, but then it turned into a blog post so I stopped and got discouraged from re-writing it. But if you're interested: http://pastie.org/4828933
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