What do square brackets mean in a type definition in Typescript

Question

In this Q/A, I have seen a union type definition wrapped into square brackets (edit: the question has been fixed by now). So far, I've only seen union types without square brackets. What is the difference between e.g.:

type UnionA = number | string;
type UnionB = [number | string];

Answer 1

I've missed a basic type of TypeScript: Tuples.

So the semantics of UnionA and UnionB are different:

let x: UnionA = 0      // works
let x: UnionA = ""     // works
let x: UnionB = 0      // doesn't work
let x: UnionB = ""     // doesn't work
let x: UnionB = [0]    // works
let x: UnionB = [""]   // works

Basically UnionB is a tuple of length 1 with its first element being a union of number | string.

In retrospect, the linked question most likely

• didn't mean to use [] to get the plain union type, or
• was intended to use , instead of | to get a meaningful tuple with multiple fields.