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().
The correct answer is using a const assertion:
function doSomething(value: "a" | "b"){}
(["a", "b"] as const).map(e => doSomething(e));
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.
constassertion to tell the compiler to infer a more specific type for["a", "b"], like this. Does that meet your needs or is there some issue with it?