Is it possible to create a union type from siblings of the same interface?
For example:
interface Foo {
values: string[];
defaultValue: string;
}
function fooo({values, defaultValue}: Foo) {}
fooo({values: ['a', 'b'], defaultValue: 'a'}); // This is a correct example
fooo({values: ['a', 'b'], defaultValue: 'c'}); // I want to avoid this case!
// Default value must be either 'a' or 'b'
// Similar to tuple type 'a' | 'b'
Can this be achieved?
Yes, it is possible to achieve this functionality using generic types.
interface CustomType<T extends number[]> {
values: [...T];
defaultValue: T[number];
}
function customFunction<T extends number[]>({values, defaultValue}: CustomType<T>) {}
The generic type T represents the tuple type of the value array. The defaultValue is constrained to only accept values specified in values with T[number].
Let's test this out:
customFunction({values: [1, 2], defaultValue: 1});
customFunction({values: [1, 2], defaultValue: 3}); // error: Type '3' is not assignable to type '1 | 2'
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