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    <p><strong>Disclaimer</strong>: This is a very formal description of the above code - this is how <em>I</em> know how to explain it. For a simpler answer - check Zirak's great answer above. This is a more in depth specification in your face and less "aha".</p> <hr> <p>Several things are happening here. Let's break it up a bit.</p> <pre><code>var arr = Array.apply(null, { length: 5 }); // Create an array of 5 `undefined` values arr.map(Number.call, Number); // Calculate and return a number based on the index passed </code></pre> <p>In the first line, <a href="http://es5.github.io/#x15.4.1" rel="noreferrer">the array constructor is called as a function</a> with <a href="http://es5.github.io/#x15.3.4.3" rel="noreferrer"><code>Function.prototype.apply</code></a>. </p> <ul> <li>The <code>this</code> value is <code>null</code> which does not matter for the Array constructor (<code>this</code> is the same <code>this</code> as in the context according to 15.3.4.3.2.a. </li> <li>Then <code>new Array</code> is called being passed an object with a <code>length</code> property - that causes that object to be an array like for all it matters to <code>.apply</code> because of the following clause in <code>.apply</code>: <ul> <li>Let len be the result of calling the [[Get]] internal method of argArray with argument "length".</li> </ul></li> <li>As such, <code>.apply</code> is passing arguments from 0 to <code>.length</code> , since calling <code>[[Get]]</code> on <code>{ length: 5 }</code> with the values 0 to 4 yields <code>undefined</code> the array constructor is called with five arguments whose value is <code>undefined</code> (getting an undeclared property of an object).</li> <li>The array constructor <a href="http://es5.github.io/#x15.4.2.1" rel="noreferrer">is called with 0, 2 or more arguments</a>. The length property of the newly constructed array is set to the number of arguments according to the specification and the values to the same values.</li> <li>Thus <code>var arr = Array.apply(null, { length: 5 });</code> creates a list of five undefined values.</li> </ul> <p><strong>Note</strong>: Notice the difference here between <code>Array.apply(0,{length: 5})</code> and <code>Array(5)</code>, the first creating five times the primitive value type <code>undefined</code> and the latter creating an empty array of length 5. Specifically, <a href="http://es5.github.io/#x15.4.4.19" rel="noreferrer">because of <code>.map</code>'s behavior (8.b)</a> and specifically <code>[[HasProperty]</code> .</p> <p>So the code above in a compliant specification is the same as:</p> <pre><code>var arr = [undefined, undefined, undefined, undefined, undefined]; arr.map(Number.call, Number); // Calculate and return a number based on the index passed </code></pre> <p>Now off to the second part.</p> <ul> <li><a href="http://es5.github.io/#x15.4.4.19" rel="noreferrer"><code>Array.prototype.map</code></a> calls the callback function (in this case <code>Number.call</code>) on each element of the array and uses the specified <code>this</code> value (in this case setting the <code>this</code> value to `Number).</li> <li>The second parameter of the callback in map (in this case <code>Number.call</code>) is the index, and the first is the this value.</li> <li>This means that <code>Number</code> is called with <code>this</code> as <code>undefined</code> (the array value) and the index as the parameter. So it's basically the same as mapping each <code>undefined</code> to its array index (since calling <a href="http://es5.github.io/#x15.7.1" rel="noreferrer"><code>Number</code></a> performs type conversion, in this case from number to number not changing the index).</li> </ul> <p>Thus, the code above takes the five undefined values and maps each to its index in the array.</p> <p>Which is why we get the result to our code.</p>
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    1. POCreating range in JavaScript - strange syntax
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    1. USAnthony Faull
    1. USBenjamin Gruenbaum
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    1. COFor docs: Specification for how map works: http://es5.github.io/#x15.4.4.19, Mozilla has a sample script that functions according to that specification at https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/map#Compatibility
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      1. USPatrick Evans
    2. COBut why does it only work with `Array.apply(null, { length: 2 })` and not `Array.apply(null, [2])` which would also call the `Array` constructor passing `2` as the length value? [fiddle](http://jsfiddle.net/wyCkU/)
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      1. USAndreas
    3. CO@Andreas `Array.apply(null,[2])` is like `Array(2)` which creates an _empty_ array of length 2 and _not_ an array containing the primitive value `undefined` two times. See my most recent edit in the note after the first part, let me know if it's clear enough and if not I'll clarify on that.
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      1. USBenjamin Gruenbaum
 

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