Function fakeAsync

packages/zone.js/lib/zone-spec/fake-async-test.ts:951–1010

(fn: Function, options: {flush?: boolean} = {})

 * @experimental
 */
export function fakeAsync(fn: Function, options: {flush?: boolean} = {}): (...args: any[]) => any {
  const {flush = true} = options;
  // Not using an arrow function to preserve context passed from call site
  const fakeAsyncFn: any = function (this: unknown, ...args: any[]) {
    const ProxyZoneSpec = getProxyZoneSpec();
    if (!ProxyZoneSpec) {
      throwProxyZoneError();
    }
    const proxyZoneSpec = ProxyZoneSpec.assertPresent();
    if (Zone.current.get('FakeAsyncTestZoneSpec')) {
      throw new Error('fakeAsync() calls can not be nested');
    }
    try {
      // in case jasmine.clock init a fakeAsyncTestZoneSpec
      if (!_fakeAsyncTestZoneSpec) {
        const FakeAsyncTestZoneSpec = Zone && (Zone as any)['FakeAsyncTestZoneSpec'];
        if (proxyZoneSpec.getDelegate() instanceof FakeAsyncTestZoneSpec) {
          throw new Error('fakeAsync() calls can not be nested');
        }

        _fakeAsyncTestZoneSpec = new FakeAsyncTestZoneSpec() as FakeAsyncTestZoneSpec;
      }

      let res: any;
      const lastProxyZoneSpec = proxyZoneSpec.getDelegate();
      proxyZoneSpec.setDelegate(_fakeAsyncTestZoneSpec);
      _fakeAsyncTestZoneSpec.lockDatePatch();
      try {
        res = fn.apply(this, args);
        if (flush) {
          _fakeAsyncTestZoneSpec.flush(20, true);
        } else {
          flushMicrotasks();
        }
      } finally {
        proxyZoneSpec.setDelegate(lastProxyZoneSpec);
      }

      if (!flush) {
        if (_fakeAsyncTestZoneSpec.pendingPeriodicTimers.length > 0) {
          throw new Error(
            `${_fakeAsyncTestZoneSpec.pendingPeriodicTimers.length} ` +
              `periodic timer(s) still in the queue.`,
          );
        }

        if (_fakeAsyncTestZoneSpec.pendingTimers.length > 0) {
          throw new Error(
            `${_fakeAsyncTestZoneSpec.pendingTimers.length} timer(s) still in the queue.`,
          );
        }
      }
      return res;
    } finally {
      resetFakeAsyncZone();
    }
  };
  fakeAsyncFn.isFakeAsync = true;