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    <p>As you said, the first part:</p> <pre><code>var arr = Array.apply(null, { length: 5 }); </code></pre> <p>creates an array of 5 <code>undefined</code> values.</p> <p>The second part is calling the <code>map</code> function of the array which takes 2 arguments and returns a new array of the same size.</p> <p>The first argument which <code>map</code> takes is actually a function to apply on each element in the array, it is expected to be a function which takes 3 arguments and returns a value. For example:</p> <pre><code>function foo(a,b,c){ ... return ... } </code></pre> <p>if we pass the function foo as the first argument it will be called for each element with</p> <ul> <li>a as the value of the current iterated element</li> <li>b as the index of the current iterated element</li> <li>c as the whole original array</li> </ul> <p>The second argument which <code>map</code> takes is being passed to the function which you pass as the first argument. But it would not be a, b, nor c in case of <code>foo</code>, it would be <code>this</code>.</p> <p>Two examples:</p> <pre><code>function bar(a,b,c){ return this } var arr2 = [3,4,5] var newArr2 = arr2.map(bar, 9); //newArr2 is equal to [9,9,9] function baz(a,b,c){ return b } var newArr3 = arr2.map(baz,9); //newArr3 is equal to [0,1,2] </code></pre> <p>and another one just to make it clearer:</p> <pre><code>function qux(a,b,c){ return a } var newArr4 = arr2.map(qux,9); //newArr4 is equal to [3,4,5] </code></pre> <p>So what about Number.call ?</p> <p><code>Number.call</code> is a function that takes 2 arguments, and tries to parse the second argument to a number (I'm not sure what it does with the first argument).</p> <p>Since the second argument that <code>map</code> is passing is the index, the value that will be placed in the new array at that index is equal to the index. Just like the function <code>baz</code> in the example above. <code>Number.call</code> will try to parse the index - it will naturally return the same value.</p> <p>The second argument you passed to the <code>map</code> function in your code doesn't actually have an effect on the result. Correct me if I'm wrong, please.</p>
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    1. CO`Number.call` is no special function that parses arguments to numbers. It is just `=== Function.prototype.call`. Only the second argument, the function that gets passed as the `this`-value to `call`, is relevant - `.map(eval.call, Number)`, `.map(String.call, Number)` and `.map(Function.prototype.call, Number)` are all equivalent.
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