How to invoke the map functional from a javascript Array

Question

Why is the map() not being invoked here?

let N = 16;
let fullSizeBufs = Array(2).map((x) =>
  Array(N).fill(-1)
)
console.log(fullSizeBufs)

We get

[undefined, undefined]

I even put a breakpoint on the Array(N).fill(-1) : it is not hit. The map() is just skipped. What is the correct syntax for this?

Thanks @Phil for putting in the `code snippet`: I'll try to pick up that little skill! — WestCoastProjects, Jun 03 '20 at 03:39
You might be interested in `Array.from({ length: N }, () => -1)` as well — Phil, Jun 03 '20 at 03:42
Does this answer your question? [JavaScript "new Array(n)" and "Array.prototype.map" weirdness](https://stackoverflow.com/questions/5501581/javascript-new-arrayn-and-array-prototype-map-weirdness) and [Most efficient way to create a zero filled JavaScript array?](https://stackoverflow.com/questions/1295584/most-efficient-way-to-create-a-zero-filled-javascript-array) Seriously everything on SO is so hard to find nowadays — user120242, Jun 03 '20 at 04:11

user120242 · Answer 1 · 2020-06-03T03:40:40.577

3

You have to fill the first array, or map has nothing to iterate on. The Array constructor initializes with references undefined, which is not the same as [undefined, undefined, undefined], which is actually an array of 3 references to undefined.
eg: [,,,].map(x=>console.log(x)) vs [undefined,undefined,undefined].map(x=>console.log(x))

let N = 16;
let fullSizeBufs = Array(2).fill(0).map((x) =>
  Array(N).fill(-1)
)
console.log(fullSizeBufs)

edited Jun 03 '20 at 03:40

answered Jun 03 '20 at 03:39

user120242

14,918
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ah ya - I'm still a bit color blind wrt javascript – WestCoastProjects Jun 03 '20 at 03:40
This is probably the most direct answer but I like the `spread operator` approach as well – WestCoastProjects Jun 03 '20 at 03:48

score 2 · Accepted Answer · edited Jun 03 '20 at 03:45

2

You could use a spread operator with the first array to make it iterable.

let N = 16;
let fullSizeBufs = [...Array(2)].map((x) =>
  Array(N).fill(-1)
)
console.log(fullSizeBufs)

edited Jun 03 '20 at 03:45

WestCoastProjects

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answered Jun 03 '20 at 03:44

Nicholas Okonneh

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I like this better than `Array(2).fill(0)` – WestCoastProjects Jun 03 '20 at 03:47
ES6 is the gift that keeps on giving. – Nicholas Okonneh Jun 03 '20 at 03:52

How to invoke the map functional from a javascript Array

2 Answers2