When attempting to use this array with a union type, TypeScript correctly identifies its type as 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 method to define the types of the array elements? If not, what other approach can be taken to address this issue without using casting in map()?
To restrict the type of an array to only accept union members, define a union type and apply a type assertion.
type AorB = 'a' | 'b';
const testArray: AorB[] = ['a', 'b'];
function doSomething(value: AorB) {...};
const test2: AorB[] = ['b', 'c'] // <- Produces errors as 'c' is not in the union.
The transition from string to AorB must be narrowed, either by specifying the array to accept only AorB, utilizing a type guard for proper narrowing, or within the receiving function itself.
The right choice is to utilize a const assertion:
function performAction(type: "a" | "b"){}
(["a", "b"] as const).map(item => performAction(item));
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