e.g. I’m trying to write a function like this:
export function fpMapSet<M extends Map<unknown,unknown>>(key: KEY_TYPE, value: VALUE_TYPE) {
Where KEY_TYPE ought to be whatever that first unknown actually is, and VALUE_TYPE should be the 2nd unknown.
And yes, I know I can write it like this:
export function fpMapSet<K,V>(key: K, value: V) {
But that’s not the question.
>Solution :
Given the question as asked, you could use conditional type inference via infer to extract the key and value types:
type MapKeyType<M> = M extends Map<infer K, any> ? K : never;
type MapValueType<M> = M extends Map<any, infer V> ? V : never;
declare function fpMapSet<M extends Map<unknown, unknown>>(
key: MapKeyType<M>, value: MapValueType<M>): void;
And you can verify that it works:
const fpMapSetStringNumber = fpMapSet<Map<string, number>>;
// const fpMapSetStringNumber: (key: string, value: number) => void
The above is an instantiation expression showing how you can set M to Map<string, number> and leave the function uncalled. You can of course call instantiate M and call the function at the same time:
fpMapSet<Map<string, number>>("abc", 123);