How to test a function which calls async function using Jasmine

Question

How to test this function:

var self = this;
self.someVariable = "initial value";
self.test = function() {
   self.functionWhichReturnsPromise().then(function() {
     self.someVariable = "new value";
   });
}

I have test case like below, I know it is wrong because before promise is resolved, the assert statement will be executed by jasmine:

it("should test the function test", function (done) {
    var abc = new ABC();
    abc.test();
    expect(abc.someVariable).toBe('new value');
});

Note that I do not want to use setTimeout() or any sleep methods.

Answer 1

Two things, you need your test function to return the Promise and you need to use an arrow function or .bind the callback to the parent function (otherwise the this in this.someVariable will refer to the callback function):

this.test = function() {
   return this.functionWhichReturnsPromise().then(() => {
     this.someVariable = "new value";
   });
}

or

this.test = function() {
   return this.functionWhichReturnsPromise().then(function() {
     this.someVariable = "new value";
   }.bind(this));
}

Then in your test you can do:

it("should test the function test", function (done) {
    var abc = new ABC();
    abc.test().then(function() {
       expect(abc.someVariable).toBe('new value');
       done();
    });
});

Answer 2

You can just spy on the function which returns the promise.

describe('when test method', function() {
  beforeEach(function() {
    this.promiseResult = Math.random();
    spyOn(ABC.prototype, 'functionWhichReturnsPromise')
      .and.returnValue(Promise.resolve(this.promiseResult));
  })

  it('then should change someVariable with the result of functionWhichReturnsPromise', function() {
    var abc = new ABC();
    abc.test();
    expect(abc.someVariable).toBe(this.promiseResult);
  });
});

No need to wait the promise, inside this unit test, you are not interested in how functionWhichReturnsPromise actually works, you only want to see that the result of calling functionWhichReturnsPromise will update someVariable value.

Have fun and luck