在 TypeScript 中键入具有联合类型的数组

Question

I'd like to use this array with a union type, but TS rightly assumes that its type is string: "Argument of type 'string' is not assignable to parameter of type '"a" | "b"'."

function doSomething(value: "a" | "b"){}

["a", "b"].map(e => doSomething(e));

Is there a ways in which I can define the types of the array elements? If not, is there another way to solve this problem? I do not want to cast it in map().

Answer 1

If you define the union type and add a type assertion, you can limit the type of that array to only accept union members.

type AorB = 'a' | 'b';
const testArray: AorB[] = ['a', 'b'];

function doSomething(value: AorB) {...};


const test2: AorB[] = ['b', 'c'] // <- Errors because 'c' is not part of the union.

The type needs to be narrowed at some point from string to AorB : either when defining the array to only accept AorB , using a type guard to narrow properly from string to AorB , or the in the receiving function.

Answer 2

The correct answer is using a const assertion :

function doSomething(value: "a" | "b"){}

(["a", "b"] as const).map(e => doSomething(e));