[Unit Testing Angular] fakeAsync

In this post, we are going to see how to use 'fakeAsync' to test async code in Angular Context.

fakeAsync using Zoom.js underhook, it detects all the async operations.

You can use the utilities function such as 

• flush: Run all the pending async function in Event loop
• flushMicrotasks: Run all the Promise Job
• tick(1000): Move time forward 1000ms 

import { fakeAsync, flush, flushMicrotasks, tick } from "@angular/core/testing";
import { of } from "rxjs";
import { delay } from "rxjs/operators";

describe("Async Testing Examples", () => {
  // Using Jasmine done function for async task
  it("Asynchronous test example with Jasmine done()", (done: DoneFn) => {
    let test = false;

    setTimeout(() => {
      console.log("running assertions");

      test = true;

      expect(test).toBeTruthy();

      done();
    }, 1000);
  });

  // running in Angular Zoom: fakeAsync
  it("Asynchronous test example - setTimeout()", fakeAsync(() => {
    let test = false;

    setTimeout(() => {});

    setTimeout(() => {
      console.log("running assertions setTimeout()");

      test = true;
    }, 1000);

    // move time forworad 1000ms
    // tick(1000);

    // exec all the timeouts
    flush();

    expect(test).toBeTruthy();
  }));

  // Microtasks: Promise based
  // there are two types of event loops
  // one is for setTimeout, http, click.... they are added into event loop
  // those might affect DOM rendering

  // another type of event loop is Microtasks, for example, Promise
  // Microtasks is more lightweight, considering it will not changing DOM

  // Exec order Call stack --> Microtasks queue --> Event queue
  it("Asynchronous test example - plain Promise", fackAsync(() => {
    let test = false;
    // Microtasks
    Promise.resolve()
      .then(() => {
        console.log("Promise first then() evaluated successfully");

        return Promise.resolve();
      })
      .then(() => {
        console.log("Promise second then() evaluated successfully");

        test = true;
      });

    flushMicrotasks();

    expect(test).toBeTruthy();
  }));

  it("Asynchronous test example - Promises + setTimeout()", fakeAsync(() => {
    let counter = 0;

    Promise.resolve().then(() => {
      counter += 10;

      setTimeout(() => {
        counter += 1;
      }, 1000);
    });

    expect(counter).toBe(0);

    flushMicrotasks();

    expect(counter).toBe(10);

    tick(500);

    expect(counter).toBe(10);

    tick(500);

    expect(counter).toBe(11);
  }));

  it("Asynchronous test example - Observables", fakeAsync(() => {
    let test = false;

    console.log("Creating Observable");

    const test$ = of(test).pipe(delay(1000));

    test$.subscribe(() => {
      test = true;
    });

    tick(1000);

    console.log("Running test assertions");
    expect(test).toBe(true);
  }));
});