online-training-oct2020/jasmine-async-await/unit-tests/js-3.6.0/spec/PromisesAndCallbacksSpec.js

describe('Promise', () => {
  it('Promise executor is run SYNCHRONOUSLY', () => {
    let executorRun = false;
    new Promise(function executor() {
      executorRun = true;
    });

    expect(executorRun).toBe(true);
  });

  it('you can resolve a promise', (done) => {
    new Promise((resolve) => setTimeout(resolve, 1))
      .then(done);
  });

  it('... or you can reject a promise', (done) => {
    new Promise((resolve, reject) => setTimeout(reject, 1))
      .then(undefined, done);
  });

  it('An error inside the executor, rejects the promise', (done) => {
    new Promise(function executor() {
      throw 'Error';
    }).catch(done);
  });

  it('you can use Promise.resolve() to wrap values or promises', (done) => {
    function iMayReturnAPromise() {
      return Math.random() >= 0.5 ? Promise.resolve() : 5;
    }

    Promise.resolve(iMayReturnAPromise()).then(done);
  });

  it('you can use Promise.resolve() to execute something just after', (done) => {
    let arr = [];
    Promise.resolve().then(() => arr.push(2));
    arr.push(1);

    setTimeout(() => {
      expect(arr).toEqual([1, 2]);
      done();
    }, 1);
  });

  /** @see https://jakearchibald.com/2015/tasks-microtasks-queues-and-schedules/ **/
  it('Promise.resolve() is normally executed before setTimeout(..., 0)', (done) => {
    let arr = [];
    setTimeout(() => arr.push('timeOut'), 0);
    Promise.resolve().then(() => {
      arr.push('resolve');
    });

    setTimeout(() => {
      expect(arr).toEqual(['resolve', 'timeOut']);
      done();
    }, 1);
  });

  it('you can create rejected promises', (done) => {
    Promise.reject('reason').catch(done);
  });
/*
  it('pay attention to "Uncaught (in promise) ..."', () => {
    Promise.reject('The error').catch(); // Outputs in the console Uncaught (in promise) The error
  });
*/

  // Chaining promises
  it('you can chain promise because .then(...) returns a promise', (done) => {
    fetch('https://jsonplaceholder.typicode.com/posts/1')
      .then(response => response.json())
      .then(json => expect(json.userId).toBe(1))
      .then(done);
  });

  it('you can use the fail callback of .then(success, fail) to handle rejected promises', (done) => {
    Promise.reject()
      .then(function success() {
        throw 'I must not be executed';
      }, function fail() {
        done();
      });
  });

  it('... or you can use .catch() to handle rejected promises', (done) => {
    Promise.reject()
      .then(function success() {
        throw 'I must not be executed';
      })
      .catch(done);
  });

  it('also .catch() returns a promise, allowing promise chaining', (done) => {
    Promise.reject()
      .catch(() => undefined)
      .then(done);
  });

  it('you must return a rejected promise if you want to execute the next fail callback', (done) => {
    function someApiCall() {
      return Promise.reject('Error');
    }

    someApiCall()
      .catch((err) => {
        console.error(err);
        return Promise.reject(err); // Without this line, .catch gets not called
      })
      .catch(done);
  });

  it('... or you can throw an error if you want to execute the next fail callback', (done) => {
    function someApiCall() {
      return Promise.reject('Error');
    }

    someApiCall()
      .catch((err) => {
        console.error(err);
        throw err; // Without this line, .catch gets not called
      })
      .catch(done);
  });

  it('values returned inside .then()/.catch() callbacks are provided to the next callback', (done) => {
    Promise.resolve(1)
      .then(value => value + 1)
      .then(value => expect(value).toBe(2));

    Promise.reject(1)
      .catch(value => value + 1)
      .then(value => expect(value).toBe(2));

    setTimeout(() => {
      done();
    }, 1);
  });


  // New await/async syntax
  it('you can use the new await/async syntax', async () => {
    function timeout(ms) {
      return new Promise((resolve) => setTimeout(resolve, ms));
    }

    const start = Date.now();
    const delay = 200;

    await timeout(delay + 1); // Just 1ms tolerance

    expect(Date.now() - start).toBeGreaterThanOrEqual(delay);
  });

  it('an async function returns a promise', (done) => {
    async function iAmAsync() {
      return 1;
    }

    iAmAsync()
      .then((val) => expect(val).toBe(1))
      .then(done);
  });

  it('await just awaits a promise resolution', async (done) => {
    await Promise.resolve();
    done();
  });

  it('await will throw an error if the promise fail', async (done) => {
    try {
      await Promise.reject();
      fail('I will not be executed');
    } catch (err) {
      done();
    }
  });

  it("Don't use new Promise(...), prefer chaining", (done) => {
    const url = 'https://jsonplaceholder.typicode.com/posts/1';

    function badlyDesignedCustomFetch() {
      return new Promise((resolve, reject) => {
        fetch(url).then((response) => {
          if (response.ok) {
            resolve(response);
          } else {
            reject('Fetch failed');
          }
        });
      });
    }

    function wellDesignedCustomFetch() {
      return fetch(url).then((response) => {
        if (!response.ok) {
          return Promise.reject('Fetch failed');
        }
        return (response);
      });
    }

    Promise.all([badlyDesignedCustomFetch(), wellDesignedCustomFetch()]).then(done);
  });


  // Parallel execution of promises
  it('you can use Promise.all([...]) to execute promises in parallel', (done) => {
    const url = 'https://jsonplaceholder.typicode.com/posts';
    const p1 = fetch(`${url}/1`);
    const p2 = fetch(`${url}/2`);


    Promise.all([p1, p2])
      .then(([res1, res2]) => {
        return Promise.all([res1.json(), res2.json()])
      })
      .then(([post1, post2]) => {
        expect(post1.id).toBe(1);
        expect(post2.id).toBe(2);
      })
      .then(done);
  });

  it('Promise.all([...]) will fail if any of the promises fails', (done) => {
    const p1 = Promise.resolve(1);
    const p2 = Promise.reject('Error');

    Promise.all([p1, p2])
      .then(() => {
        fail('I will not be executed')
      })
      .catch(done);
  });

  it("if you don't want Promise.all() to fail, wrap the promises in a promise that will not fail", (done) => {
    function iMayFail(val) {
      return Math.random() >= 0.5 ? Promise.resolve(val) : Promise.reject(val);
    }

    function promiseOr(p, value) {
      return p.then(res => res, () => value);
    }

    const p1 = iMayFail(10);
    const p2 = iMayFail(9);

    Promise.all([promiseOr(p1, null), promiseOr(p2, null)])
      .then(([val1, val2]) => {
        expect(val1 === 10 || val1 === null).toBe(true);
        expect(val2 === 9 || val2 === null).toBe(true);
      })
      .catch(() => {
        fail('I will not be executed')
      })
      .then(done);
  });

  it('Promise.race([...]) will resolve as soon as one of the promises resolves o rejects', (done) => {
    const timeout = new Promise((resolve, reject) => setTimeout(reject, 100));
    const data = fetch('https://jsonplaceholder.typicode.com/posts/1');

    Promise.race([data, timeout])
      .then(() => console.log('Fetch OK'))
      .catch(() => console.log('Fetch timeout'))
      .then(done);
  });
});