TypeScript array with minimum length

Question

How can you create a type in TypeScript that only accepts arrays with two or more elements?

needsTwoOrMore([\"onlyOne\"]) // should have error
needsTwoOrMore         


        

Answer 1

type FixedTwoArray<T> = [T,T]
interface TwoOrMoreArray<T> extends Array<T> {
    0: T
    1: T
}

let x: FixedTwoArray<number> = [1,2];
let y: TwoOrMoreArray<string> = ['a','b','c'];

TypeScript Playground

Answer 2

This is an old question and the answer is fine (it helped me as well), but I just stumbled across this solution as well while playing.

I had already defined a typed Tuple (type Tuple<T> = [T, T];), and then below that, I had defined array of two or more as described above (type ArrayOfTwoOrMore<T> = { 0: T, 1: T } & T[];).

It occurred to me to try using the Tuple<T> structure in place of { 0: T, 1: T }, like so:

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

And it worked. Nice! It's a little more concise and perhaps able to be clearer in some use-cases.

Worth noting is that a tuple doesn't have to be two items of the same type. Something like ['hello', 2] is a valid tuple. In my little code snippet, it just happens to be a suitable name and it needs to contain two elements of the same type.

Answer 3

This can be accomplished with a type like:

type ArrayTwoOrMore<T> = {
    0: T
    1: T
} & Array<T>

declare function needsTwoOrMore(arg: ArrayTwoOrMore<string>): void

needsTwoOrMore(["onlyOne"]) // has error
needsTwoOrMore(["one", "two"]) // allowed
needsTwoOrMore(["one", "two", "three"]) // also allowed

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