What it type `[[string, number]]` vs `[string, number][]`

[unional]

var items: [[string, number]];
for (let item of items) { // item: [string, number]
   let name = item[0];  // name: string
   let quantity = item[1]; // quantity: number
}

var items: [string, number][];
for (let item of items) { // item: [string, number]
   let name = item[0];  // name: string
   let quantity = item[1]; // quantity: number
}

They look the same but can't assign to each other:

var a: [[string, number]];
var b: [string, number][];
a = b.map(c => c); // Error: Type '[string, number][]' is not assignable to type '[[string, number]]'.  Property '0' is missing in type '[string, number][]'
a = b.map(c => [c[0], c[1]]); // Error: Type '(string | number)[][]' is not assignable to type '[[string, number]]'. Property '0' is missing in type '(string | number)[][]'
b = a.map(c => c); // OK
b = a.map(c => [c[0], c[1]]); // Error: Type '(string | number)[][]' is not assignable to type '[[string, number]]'. Type '(string | number)[]' is not assignable to type '[string, number]'. Property '0' is missing in type '(string | number)[]'

[DanielRosenwasser]

[[string, number]] is a single-element tuple where the element has the type [string, number].

[string, number][] is an array of [string, number] tuples.

@unional, I think questions such as these would be better fit for either StackOverflow or our Gitter room.

[unional]

Thanks. Didn't know gitter room exist. Will move questions like these over there.

[DanielRosenwasser]

No worries - we only recently got the Gitter room so it's easy to miss.