Check that array of tuples (2d array) has at least 1 item in Typescript

Question

I have a simple type check in my Typescript variable declaration that ensures all elements in an array of tuples are of length 2 and are both numbers:

const testArea: [number, number][] = [[0, 0], [0, 220000], [220000, 220000], [220000, 0]];

This allows me to correctly initialise the variable, however it still allows an empty array:

const testArea: [number, number][] = []; // No error

I have tried to re-write this as:

const testArea: [[number, number]] = [[0, 0], [0, 220000], [220000, 220000], [220000, 0]]; // error: only allows a single tuple in the array

but that didn't work for the primary case of many tuples in the array (though it does seem to prevent an empty array from being valid).

How can I check that the outer array contains tuples of type [number, number] and also that it contains at least 1 of these tuples?

Answer 1

You just need to use small trick with rest operator

type Tuple = [number, number]

type NonEmptyArray<T> = [T, ...T[]];

type Data = NonEmptyArray<Tuple>

const data: Data = [] // error
const data1: Data = [[]] // error
const data2: Data = [[1]] // error
const data2: Data = [[1,1]] // Ok

There are many alternatives to make sure the array is not empty,

here is the other one:

type NonEmptyArray2<T extends unknown[]> = T['length'] extends 0 ? never : T

You can take a look on @jcalz's answer here