Why is the output [ 0, 1, 2, 3, 4 ] for this function on the following array?

Question

var hello = Array.apply(null, Array(5)).map(function (x, i) { return i; });

I get what map does, but I don't understand why the function does what it does to make the hello == [0, 1, 2, 3, 4]

It is keys of the array of length 5. What makes you confused? — Vladimir G., May 25 '17 at 08:59
`Array.apply(null, Array(5))` returns an array of size full of `undefined`s. Whatever there is inside, this `map` function ignores values and converts it into array [0, 1, 2, 3, 4] because second argument of `map` is index. — Yeldar Kurmangaliyev, May 25 '17 at 09:00
Also have a look at https://stackoverflow.com/questions/22949976/why-does-array-applynull-args-act-inconsistently-when-dealing-with-sparse-a — Jonas Wilms, May 25 '17 at 09:01

score 0 · Answer 1 · answered May 25 '17 at 08:59

0

The second parameter on the closure passed to map is the array key. That is what is being returned so you're getting 5 array keys, starting at 0.

answered May 25 '17 at 08:59

Jim Wright

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Davin Tryon · Answer 2 · 2017-05-25T09:26:29.117

0

Maybe we should break it down:

var arr = Array(5);
arr === [,,,,,];  // array with five undefined slots
arr === [undefined, undefined, undefined, undefined, undefined]; // same as this :)

Each has an index:

arr[4]; // yay! => undefined

map loops through each slot and the mapping function gets 2 params, the value and the index.

function (x, i) { return i; }

map produces an array of all the return values from the mapping function.

edited May 25 '17 at 09:26

answered May 25 '17 at 09:03

Davin Tryon

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_"// array with five undefined slots"_ `[,,,,,]` has more than five elements :) – Andreas May 25 '17 at 09:14
@Andreas I don't understand what you mean. :) `Array(5) === [,,,,,]` right? – Davin Tryon May 25 '17 at 09:18
`JSON.stringify(Array(5)) -> "[null,null,null,null,null]"` compare the number of commas. There is one too many in your example. – Andreas May 25 '17 at 09:23
@DavinTryon I think Andreas means that an empty array of length 5, would probably only have 4 commas, not 5 ;) – Brett East May 25 '17 at 09:23
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@Andreas did you test your theory? It is JS afterall :) The literal `[,,,,]` only has `undefined x 4` :) – Davin Tryon May 25 '17 at 09:24

Nick is tired · Answer 3 · 2017-05-25T09:10:01.830

You can find out by performing a dry run of your code:

var hello = Array.apply(null, Array(5))

At this point:

hello == [undefined,undefined,undefined,undefined,undefined]

Now:

hello.map(function (x, i) { return i; });

Is the equivalent to (use the comments to see how the below references the above function:

for(var i = 0; i < hello.length; i++){ // i is the index
    x = hello[i];                      // x is the value at each index

    // Function body below using `hello[i] = ` instead of return
    hello[i] = i;
}

Hence the result from setting each element in the list to the index of that element:

[0, 1, 2, 3, 4]

Why is the output [ 0, 1, 2, 3, 4 ] for this function on the following array?

3 Answers3