How to type N array of strings with TypeScript?

July 24, 2022

I have an array like this:

const rows: Row[] = [
  [
    1,
    ["breakfast", "apples"],
  ],
  [
    2,
    ["lunch", "burguer"],
    ["dinner", "pasta"],
  ],
];

The array can have any number of items, each one is an array itself with the first item always being a number and the rest being arrays of 2 strings.

This is the type I came up with:

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

But it’s not working, it says that the 2nd item should be a string instead of an array of strings:

Type ‘[string, string]’ is not assignable to type ‘string’.

Here’s a playground with an example.

>Solution :

A rest element in a tuple type should be of the form [...YourDesiredElementType[]]. You want elements to be [string, string], so the rest element looks like ...[string, string][]:

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

And then things work as desired:

const rows: Row[] = [
  [
    1,
    ["bananas", "apples"],
    ["steak", "burguer"],
  ],
  [
    2,
    ["orange", "lemon"],
    ["pizza", "pasta"],
  ],
]; // okay

Playground link to code