an ofType() implementation that propagates types better

oftype.ts

import {Action} from '@ngrx/store';  // from //third_party/javascript/ngrx:ngrx_store
import {OperatorFunction} from 'rxjs';
import {filter} from 'rxjs/operators';

// Removes types from T that are not assignable to U.
export type Filter<T, U> = T extends U ? T : never;

/**
 * Equivalent to the `ofType` operator provided by @ngrx/effects, but with
 * smarter type inference.
 *
 * Type propagation currently scales to five input strings.
 *
 * See https://github.com/ngrx/platform/blob/master/docs/effects/api.md#oftype
 */
export function ofType<
    T1 extends string, U extends Action, V extends Filter<U, {type: T1}>>(
    t1: T1,
    ): OperatorFunction<U, V>;
export function ofType<T1 extends string, T2 extends string, U extends
                           Action, V extends Filter<U, {type: T1 | T2}>>(
    t1: T1,
    t2: T2,
    ): OperatorFunction<U, V>;
export function
    ofType<T1 extends string, T2 extends string, T3 extends string, U extends
               Action, V extends Filter<U, {type: T1 | T2 | T3}>>(
        t1: T1,
        t2: T2,
        t3: T3,
        ): OperatorFunction<U, V>;
export function ofType<T1 extends string, T2 extends string, T3 extends
                           string, T4 extends string, U extends Action, V
                               extends Filter<U, {type: T1 | T2 | T3 | T4}>>(
    t1: T1,
    t2: T2,
    t3: T3,
    t4: T4,
    ): OperatorFunction<U, V>;
export function
    ofType<T1 extends string, T2 extends string, T3 extends string, T4 extends
               string, T5 extends string, U extends Action, V extends
                   Filter<U, {type: T1 | T2 | T3 | T4 | T5}>>(
        t1: T1,
        t2: T2,
        t3: T3,
        t4: T4,
        t5: T5,
        ): OperatorFunction<U, V>;
export function ofType<U extends Action>(...allowedTypes: string[]):
    OperatorFunction<U, U> {
  return filter(
      (action: U): boolean => allowedTypes.some(type => type === action.type));
}