Why does [...Array(3)] gives a different result than Array(3).map(el => undefined)

Asked Mar 15 '21 at 22:49

Active Mar 16 '21 at 12:03

Viewed 41 times

In javascript, when creating a new Array of length 3, I cannot seem to wrap my head around why this works:

[...Array(3)] // [undefined, undefined, undefined]

But this won't work:

Array(3).map(e => undefined) // [ , , ]

As I understood correctly here, Array(3) will return an array with 3 empty slots, which is not the same as [undefined, undefined, undefined], but why can you not iterate over them with map but the spread operator can?

edited Mar 16 '21 at 12:03

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asked Mar 15 '21 at 22:49

dreadnaught

4

[].map() skips empty slots, look at a polyfill's source for details. – dandavis Mar 15 '21 at 22:52
Ok! thanks, so the spread operator does not skip empty slots? – dreadnaught Mar 15 '21 at 22:57
4

spread doesn't skip empty (uninitialized) elements. – dandavis Mar 15 '21 at 23:01

Why does [...Array(3)] gives a different result than Array(3).map(el => undefined)

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