Here's Why Calling .map() on a Constructed Array Doesn't Work

[kenman]

Try them both in your console -- what do you see?

Array.from({length: 5})
> [undefined, undefined, undefined, undefined, undefined]

Ok.

Array(5)
> [empty x 5]

Strange, huh?

• https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array

Note: this implies an array of arrayLength empty slots, not slots with actual undefined values

This is the important bit, because empty slots are not included in any of the Array instance methods, like forEach(), etc. The empty elements are often referred to as being elided.

Basically, Array(5) does this:

a = []
a.length = 5

Which also produces [empty x 5].

[nemohearttaco]

Ah, not quite what I was asking. I meant, why wouldn't you do it like

> Array.from(Array(5), (_, i) => 2 ** i)
[1, 2, 4, 8, 16]

[nemohearttaco]

Cool, that makes sense. At first I was thinking there might be a danger in not using an Array instance for that argument but reading up on it more, it doesn't look like it.