Random useful TypeScript code snippets

random-typescript.ts

/*
Ever need just the keys of an interface who's values match a certain type? e.g.
interface A {
   id: string;
   other: boolean:
}
we want a union of the keys of the properties with a value of type string (useful for strongly typing a mapping arrays of objects, to hashmap with a specific key value to be used as key in hashmap)
*/

type Unionize<T extends object> = {
  [k in keyof T]: {k: k; v: T[k]}
}[keyof T];

type PickKeysWithTypeV<T extends object, V> =
  Extract<Unionize<T>, {v: V}>['k'];

function mapFromArray<T extends {}>(array: T[], propName: PickKeysWithTypeV<T, PropertyKey>) {
  return array.reduce((prev, curr: T) => {
    const key = curr[propName];
    prev.set(key, curr);
    return prev;
  }, new Map<T[PickKeysWithTypeV<T, PropertyKey>], T>());
}