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
– WestCoastProjects, Commented Jun 3, 2020 at 3:39
You might be interested in Array.from({ length: N }, () => -1) as well — Phil
– Phil, Commented Jun 3, 2020 at 3:42
Does this answer your question? JavaScript "new Array(n)" and "Array.prototype.map" weirdness and Most efficient way to create a zero filled JavaScript array? Seriously everything on SO is so hard to find nowadays — user120242
– user120242, Commented Jun 3, 2020 at 4:11

user120242 · Accepted Answer · 2020-06-03 03:40:40Z

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 3, 2020 at 3:40

answered Jun 3, 2020 at 3:39

user120242

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2 Comments

WestCoastProjects Over a year ago

ah ya - I'm still a bit color blind wrt javascript

WestCoastProjects Over a year ago

This is probably the most direct answer but I like the spread operator approach as well

WestCoastProjects · Accepted Answer · 2020-06-03 03:45:55Z

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 3, 2020 at 3:45

WestCoastProjects

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answered Jun 3, 2020 at 3:44

Nicholas Okonneh

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2 Comments

WestCoastProjects Over a year ago

I like this better than Array(2).fill(0)

Nicholas Okonneh Over a year ago

ES6 is the gift that keeps on giving.

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