Constructing Arrays using apply() vs new Array

Question

I want to initialize an array of a given size with elements set to undefined by default.

new Array(5) returns an array of 5 empty items. Array.apply(null, Array(5)) returns exactly what I want: a five element array with each element as undefined.

Why does the second option produce a different array value than the first?

This distinction is important because I can map over an array of empty items but I can for undefined elements.

new Array(5).map(Boolean) // [false, false, false, false, false]

Array.apply(null, Array(5)) // [ <5 empty items>]

Possible duplicate of What is Array.apply actually doing

skyline3000
– skyline3000

2019-08-21 02:23:58 +00:00
Commented Aug 21, 2019 at 2:23 — skyline3000
– skyline3000, Commented Aug 21, 2019 at 2:23

Stephen Collins · Accepted Answer · 2019-08-21 02:34:11Z

1

.map is a native Array object method that is expected to return a value for every iteration over the array to create a new copied array from the array. Boolean can be used as a constructor function to create a boolean object with a true or false value. Try the same thing with Number like:

new Array(5).map(Number)

In your second example, .apply calls a function with a supplied context value as the first parameter, and the parameter the function takes in as the second parameter. Also here, Array can be used as a constructor function, to construct a new Array (just like with Number and Boolean of their respective types)

edited Aug 21, 2019 at 2:34

answered Aug 21, 2019 at 2:29

Stephen Collins

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kamprasad · Accepted Answer · 2019-08-21 02:46:40Z

0

First of All, Javascript array is a Object. If you check this, you will understand it. ONly specialty is,it has length property that regular object in javascript.

var array = new Array(5) 
typeof array

And other thing is Array.apply mean, it will call array native function with setting given context which is null for your case and parameters are in another array.

Array.apply(null, [,,,,,]);

output will be [undefined, undefined, undefined, undefined, undefined]

answered Aug 21, 2019 at 2:46

kamprasad

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Adrian Brand · Accepted Answer · 2019-08-21 02:47:14Z

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I personally would use Array.from

console.log(Array.from({ length: 5 }))

// if you want to initialise the values you can add a constructor function

console.log(Array.from({ length: 5 }, (_, index) => index + 1))

answered Aug 21, 2019 at 2:47

Adrian Brand

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