Typescript: Array with arrays fails when using map

Question

type Point = [number, number]
type Points = Array<Point>

const ok: Points = [[0, 0]]
const fail: Points = [0].map(() => [0, 0])

Type 'number[][]' is not assignable to type '[number, number][]'.

Any ideas?

Playground

This works though const fail: Points = [0].map(() => <Point>[0, 0]) Playground — adiga
– adiga, Commented Feb 28, 2019 at 13:45
I think you need to explicitly define the map return type: const fail: Points = [0].map<Point>(() => [0, 0]) Otherwise, map cannot exactly guess what the return type will be, since it's dynamic. typescriptlang.org/play/… — briosheje
– briosheje, Commented Feb 28, 2019 at 13:46
Yea, but briosheje’s suggestion works. Thanks anyway. You should make that an answer @briosheje — David Hellsing
– David Hellsing, Commented Feb 28, 2019 at 13:52

Sergiu Paraschiv · Accepted Answer · 2019-02-28 13:56:27Z

4

It's because of how map is typed.

The "official" definition is:

map<U>(callbackfn: (value: T, index: number, array: T[]) => U, thisArg?: any): U[];

If you don't specify U then default U[] will be [] (same as Array).

The problem is that [number, number] is a subset of Array AND map without specifying U would mean the return type will be Array.

The two types not being compatible, you get that error.

Solution is to specify what map is allowed to return:

const okToo: Points = [0].map<Point>(() => [0, 0])

On the other hand I'd expect this to fail too:

const notOk: Points = [0].map<Point>(() => [0, 0, 0])

but on TypeScript 2.6.1 it's allowed. It's an error in 3.3 though, which is really nice.

edited Feb 28, 2019 at 13:56

answered Feb 28, 2019 at 13:51

Sergiu Paraschiv

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6 Comments

briosheje Over a year ago

Courtesy playground: typescriptlang.org/play/…

Sergiu Paraschiv Over a year ago

@briosheje thanks! Question though: how do I use that to see the compile errors?

briosheje Over a year ago

whenever you type something wrong, you just need to hightlight the row and it will show the error, like the original playground the OP provided: prntscr.com/mrcywk (playground here: typescriptlang.org/play/…)

Sergiu Paraschiv Over a year ago

Cool! And it looks like in TS3.3 [0, 0, 0] is an error too.

briosheje Over a year ago

Well, I guess it should be an error, since you're explicitly defining the return type (which must be of type "Point"). It's quite strange that it's allowed in 2.6.1, to me. Interesting :P.

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