CODEWITHSUNDEEP
arraystypestypescripttuples
Jim Hunziker
Jim Hunziker

Reputation: 15390

Alternate ways to declare arrays in TypeScript

My Typescript file will compile with either of these declarations. Are they equivalent? Is there a name for these ways of declaring arrays?

interface Foo {
    bounds: number[][];
    bounds2: [[number, number], [number, number]];
}

The library into which I'm passing bounds takes an array of two two-element arrays. bounds2 seems to express that more clearly above, but I'm not sure it's really doing the same thing.

Upvotes: 4

Views: 463

Answers (2)

MartyIX
MartyIX

Reputation: 28656

I think the following code illustrates it quite nicely:

let a: number[][];
let b: [[number, number], [number, number]];

a = [5]; // error
a = [[5]]; // ok
a = [[5],[8,9,10]]; // ok
a = [[5],[8,9,10], [5]]; // ok
a = [[5, 'a']]; // error

b = [5]; // error
b = [[5]] // error
b = [[5,5]] // error
b = [[5,5], [5,5]] // ok
b = [[5,5],  [5]] // error
b = [[5,5],  [5,5,5]] // ok, a bit surprisingly
b = [[5,5,5],  [5,5,5]] // ok, a bit surprisingly
b = [[5,5,5],  [5,5,5], [5,5,5]] // ok, a bit surprisingly
b = [[5,5,5],  [5,5,5], [5,5,5, 'a']] // error
b = [[5,5,5],  [5,5,5], [5,5,5], [5,5,5,5,5,5]] // ok, a bit surprisingly 
b = [[5,5,5]] // error

Upvotes: 1

Kuba Jagoda
Kuba Jagoda

Reputation: 5557

It's not the same thing.

bounds represents an array of arrays of numbers.

bounds2 represents something that is called a tuple type, which is like a more detailed array description (with types on specified indexes). In this case this is a tuple containing two tuples, which both contain two numbers

Upvotes: 1

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