Difference between new Array(7) and Array.apply(null, Array(7))

Question

Anyone can explain me difference between new Array(7) and Array.apply(null, Array(7))? In context:

Array.apply(null, Array(7)).map((e, i) => {
  return moment(i, 'e').format('ddd');
}); // ["Sun" ,"Mon", "Tue", "Wed", "Thu", "Fri", "Sat"]

new Array(7).map((e, i) => {
  return moment(i, 'e').format('dd');
}); // [empty × 7]

<script src="https://cdnjs.cloudflare.com/ajax/libs/moment.js/2.20.1/moment-with-locales.min.js"></script>

I'm getting an error when that codes runs. `"TypeError: (new Array(...)).apply is not a function",` — user184994, Dec 28 '17 at 22:18
Possible duplicate of [JavaScript "new Array(n)" and "Array.prototype.map" weirdness](https://stackoverflow.com/questions/5501581/javascript-new-arrayn-and-array-prototype-map-weirdness) — user184994, Dec 28 '17 at 22:22
check the link in the comment above. The top answer goes into exactly why this happens — user184994, Dec 28 '17 at 22:25
no it's not I wanna know why Array.apply() work and new Array() doesn't work, why we should pass parameters(null and Array7)? — Rami Chasygov, Dec 28 '17 at 22:28

Spencer Wieczorek · Answer 1 · 2017-12-28T22:39:02.177

Array.apply calls the prototype method on the Array object. Where as doing Array(7) is creating a new array of seven elements, as such new Array(7).apply is not a valid call and would give the following error:

Uncaught TypeError: (intermediate value).apply is not a function

Doing Array.apply(null, Array(7)) is the only valid call here.

Edit

The statement Array.apply(null, Array(7)) will create an array populated with undefined elements. As such calling .map() will iterate those elements and produce your expected result.

On the other hand calling just new Array(7) will create a sparse array with a define length of 7. Meaning the length will be defined as 7 but it didn't populate any elements into the array. So there is nothing to iterate over yet.

prototype method it is like static method? – Rami Chasygov Dec 28 '17 at 22:22 — Rami Chasygov, Dec 28 '17 at 22:22

score 3 · Accepted Answer · answered Dec 28 '17 at 22:26

 Array(7)

creates a Sparse array with no elements set. So as an object it would look like:

 {length: 7}

If you would call any of the new cool array methods, it wouldnt iterate as there are no elements in the array. However if that is spread into the Array constructor:

  Array(...Array(3))

Its equal to

 Array(undefined, undefined, undefined)

And that actually creates 3 (or 7) undefined array slots:

 {
 0:undefined,
 1:undefined,
 2:undefined,
 length:3
 }

And now you can iterate over it with map etc. The same is possible with:

 Array(7).fill().map(func)

and

 Array.from({length:7}, func);

score 1 · Answer 3 · answered Dec 28 '17 at 22:12

1

these two approach are equivalent since Function.prototype.apply is part of the Function api.

it looks you likely need something like:

Array
  .from({ length: 7 }, (_, i) => moment(i, 'e').format('ddd'))
;

answered Dec 28 '17 at 22:12

Hitmands

13,491
4
34
69

Difference between new Array(7) and Array.apply(null, Array(7))

3 Answers3