Jasmine - how to spyOn instance methods

Question

I have a function

var data = {};
var myFunc = function() {
  data.stuff = new ClassName().doA().doB().doC();
};

I'd like to test that doA, doB, and doC were all called.

I tried spying on the instance methods like this

beforeEach(function() {
  spyOn(ClassName, 'doA');
};
it('should call doA', function() {
  myFunc();
  expect(ClassName.doA).toHaveBeenCalled();
});

but that just gives me a "doA() method does not exist" error.

Any ideas?

Solution 1

Where you went wrong was your understanding of how to refer to methods in JavaScript in a static context. What your code is actually doing is spying on ClassName.doA (that is, the function attached to the ClassName constructor as the property doA, which is not what you want).

If you want to detect when that method gets called on any instance of ClassName anywhere, you need to spy on the prototype.

beforeEach(function() {
  spyOn(ClassName.prototype, 'doA');
});
it('should call doA', function() {
  myFunc();
  expect(ClassName.prototype.doA).toHaveBeenCalled();
});

Of course, this is assuming that doA lives in the prototype chain. If it's an own-property, then there is no technique that you can use without being able to refer to the anonymous object in myFunc. If you had access to the ClassName instance inside myFunc, that would be ideal, since you could just spyOn that object directly.

P.S. You should really put "Jasmine" in the title.

Solution 2

Let’s do some code refactoring as we want implement constructor injection pattern as James Shore mentions that:

Dependency injection means giving an object its own instance variables. Really. That’s it.

var data = {};
var stuff = new ClassName()

var myFunc = function(stuff) { // move step of creation new object outside
  data.stuff = stuff.doA().doB().doC();
};

And time for some tests

function ClassName() {
}

var data = {};
var stuff = new ClassName()

var myFunc = function(stuff) {
  data.stuff = stuff.doA().doB().doC();
};


describe('stub for ClassName implementation', function() {
  var stubStuff = {
    doA: function() {
      return stubStuff
    },
    doB: function() {
      return stubStuff
    },
    doC: function() {
      return stubStuff
    }
  }

  beforeEach(function() {
    spyOn(stubStuff, 'doA').and.callThrough();
  });

  it('calls "doA" on "myFunc" exection', function() {
    myFunc(stubStuff);
    expect(stubStuff.doA).toHaveBeenCalled();
  });
});

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