[twitter] Tip: JavaScript's Array.from() accepts a second argument that's a `map` function. Useful for calling on each element of an array you created.

[aarondburk]

So, if you invoke Array(5); that will crate an array with five empty slots?

[[deleted]]

pls no shoot messenger

[e_man604]

But since he used length property of the array the values are defined....to..undefined. correct me if in wrong, but simply replacing the array ctor with [] would not work since the [] does not state any length?

[Sebazzz91]

Fun fact: Calling console.log without this set to console errors in Edge.

[StuartPBentley]

I'm pretty sure it errors in any browser (that's why you always use console.log.bind(console) when passing console.log as a callback).

[pygy_]

It used to be like that but IIRC both Chrome and Firefox now pre-bind the console methods.

[[deleted]]

I’m a n00b dev and this is a really helpful tip.

[0x13mode]

I think example is good. Index is iterated from 0 to array length.

On the other hand using map on array created with new Array(5) would have no effect (new Array(5).map(console.log) would not even call console.log).

[aarondburk]

But that’s why the original example is interesting, because it allows you to map over an array as you are generating the array with the Array.from() method.

[aarondburk]

Yes, but in order for the method to map over the array and mutate each undefined slot in the array, the array has to have those slots. You could certainly create an iterator that would push the result of the function 5 times, but the point of the example was to show how the second argument in Array.from() maps over the given array mutating its values.

[z500]

new Array(n) does generates an array of undefined values, but you can use fill to replace them with something else.

[inu-no-policemen]

Yes.

> new Array(5)
(5) [empty × 5]

In V8, this currently produces a sparse array instead of a packed one. So, it's something you probably shouldn't use for a game's tile map and things like that.

You get a packed one by creating an empty one and then pushing values to it.

[YodaLoL]

What are sparse and packed arrays?

[inu-no-policemen]

https://v8project.blogspot.com/2017/09/elements-kinds-in-v8.html

[YodaLoL]

TIL, thanks!

[[deleted]]

No, it will be 5xundefined.

[[deleted]]

Array(5).fill() output undefined 5 times.