Typing a tuple of N numbers followed by a single string

user_13088508 0
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Consider an array type like the following:

let example1: MyArray = ['John'],
    example2: MyArray = [4, 5, 1, 5, 'Eric'],
    example3: MyArray = [1, 5, 7, 3, 4, 5, 1, 'Joe'],
    ...

So one string in the end and an arbitrary count of numbers.

So far I can imagine a naive implementation of the MyArray type (which looks like the Death by a Thousand Overloads problem):

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

This is obviously suboptimal. Do better approaches exist?

typescript-types typescript
2 Answers

Yes, you can type this, though only in Typescript 4.2 or later: Leading/Middle Rest Elements in Tuple Types.

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

const example1: MyArray = ['John'],
      example2: MyArray = [4, 5, 1, 5, 'Eric'],
      example3: MyArray = [1, 5, 7, 3, 4, 5, 1, 'Joe'];

Prior to that, 3.0 introduced Rest elements in tuple types, but at that time, rest elements were limited to the end of the tuple. So you could do [string, ...number[]], but not [...number[], string].

user_7784300

Typescript allows for

type MyArray = [number, ...string[]]
const arr: MyArray = [12, 'wsd', 'qwdqwdqd', 'qwqdqdqdw'];

Unfortunately type MyArray = [...string[], number] is not allowed. TS says A rest element must be last in a tuple type.

user_143005930
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