Utility types for immutable js with typescript

typescript-immutablejs.d.ts

// import types: https://www.typescriptlang.org/docs/handbook/release-notes/typescript-2-9.html#import-types
// allow us to import types on global type definitions
// To use, eg, for a map of User:
// type UserMap = ImmutableMap<User>;
interface ImmutableMap<T> extends import('immutable').Map<string, any> {
  get<K extends keyof T>(name: K): T[K];
}

type ImmutableMapList<T> = import('immutable').List<ImmutableMap<T>>;

type ImmutableUtilType<T = {}> = ImmutableMap<T> | ImmutableMapList<T>;

// eg: if we have
// const user: User = someuser;
// const foo: ImmutableMap<User> = Map(user);
// then we can get the User type back using from Immutable like:
// const sameUser: FromImmutable<typeof foo> = foo.toJS();
// sameUSer will be of type User
type FromImmutable<T> = T extends ImmutableMap<infer U> ? U : T extends ImmutableMapList<infer V> ? V[] : never;

// This type takes an interface and if any of it's types extends a ImmutableType we unpack it
// Using directly ImmutableUtilType and FromImmutable doesn't seem to work everytime for some reason for which
// I currently don't have the time to research
// TODO: dry 
type FromImmutableToJS<T> = {
  [P in keyof T]: T[P] extends ImmutableMapList<infer U> ? U[] : T[P] extends ImmutableMap<infer V> ? V : T[P]
};