CODEWITHSUNDEEP
typescripttypescript-typings
modulitos
modulitos

Reputation: 15814

How to type a dynamically sized array in Typescript?

I want to type an array where the first element is always a number, and the rest of the elements are always strings, and the length as to be at least 1.

Here is my best effort:

type MyArray =
  | [number]
  | [number, string]
  | [number, string, string]
  | [number, string, string, string];

How do I make that go on forever?

Upvotes: 2

Views: 1557

Answers (1)

jcalz
jcalz

Reputation: 327954

Before TypeScript 3.0 there really wasn't anything better than a union like you have. But then a bunch of related new features were introduced to better support tuple types, and specifically their relationship to lists of function parameters. And since functions support a final rest parameter representing the type of an indefinite number of parameters as an array, it made sense to introduce a final rest element in a tuple representing the type of an indefinite number of tuple elements as an array.

Hey, in fact, your use case is explicitly mentioned as an example in the documentation:

For example, [number, ...string[]] means tuples with a number element followed by any number of string elements.

So let's try that:

type MyArray = [number, ...string[]];
const okay0: MyArray = [0]; // okay
const okay1: MyArray = [1, "a"]; // okay
const okay2: MyArray = [2, "a", "b"]; // okay
const okay3: MyArray = [3, "a", "b", "c", "d", "e", "f", "g", "h", "i", "j"]; // okay

const bad0: MyArray = [0, "a", false]; // error!
//    ~~~~ <-- boolean is not assignable to string
const bad1: MyArray = ["x", "y"]; // error!
//                     ~~~ <-- string is not assignable to number
const bad2: MyArray = []; // error!
//    ~~~~ <--- property '0' is missing (i.e., bad2[0] is missing)

Looks good to me. Hope that helps; good luck!

Link to code

Upvotes: 2

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