Improved NgRx type safety with TypeScript 2.8

actions-union.ts

type FunctionType = (...args: any[]) => any;

interface ActionCreatorsMapObject {
  [action: string]: FunctionType;
}

export type ActionsUnion<A extends ActionCreatorsMapObject> = ReturnType<A[keyof A]>;

create-action.ts

export interface Action<T extends string> {
  type: T;
}

export interface ActionWithPayload<T extends string, P> extends Action<T> {
  payload: P;
}

export function createAction<T extends string>(type: T): Action<T>;
export function createAction<T extends string, P>(type: T, payload: P): ActionWithPayload<T, P>;
export function createAction<T extends string, P>(type: T, payload?: P) {
  return payload === undefined ? {type} : {type, payload};
}

of-type.ts

// https://github.com/ngrx/platform/issues/860
type ActionsOfType<A, T extends string> = A extends Action<T> ? A : never;

export function ofType28<V, T1 extends string>(t1: T1): OperatorFunction<V, ActionsOfType<V, T1>>;
export function ofType28(...types: string[]) {
  return (source: Observable<any>) => {
    return source.pipe(filter((action) => types.indexOf(action.type) !== -1)) as any;
  };
}

override.ts

// DO NOT USE ON PRODUCTION

// import {Action} from '@ngrx/store';
//
// declare module "@ngrx/store" {
//   export interface Action<T extends string = string> {
//       type: T;
//   }
// }
//
// declare module "@ngrx/effects" {
//   export function ofType<V, T1 extends string>(t1: T1): OperatorFunction<V, ActionsOfType<V, T1>>;
// }

ts28.ts

// T extends U ? X : Y

type TypeName<T> =
	T extends string ? "string" :
	T extends number ? "number" :
	T extends boolean ? "boolean" :
	T extends undefined ? "undefined" :
	T extends Function ? "function" :
	"object";

type T0 = TypeName<string>;  // "string"
type T1 = TypeName<"a">;  // "string"

type T2 = ReturnType<() => string>;  // "string"