How I can I unit test that a function inside another function was called?

Question

How I can I unit test that a function inside another function was called? I can't change the source code, so I need to test this as-is.

How can I do this? Here's my code:

function B(){ console.log('function b'); }

function A(){
    B();
}  

Jasmine test:

it('should check function B in function A was called', function () {
    spyOn(window, 'B');
    A();
    expect(B).toHaveBeenCalled();
});

Answer 1

Spies

Jasmine has test double functions called spies. A spy can stub any function and tracks calls to it and all arguments. A spy only exists in the describe or it block in which it is defined, and will be removed after each spec. There are special matchers for interacting with spies. This syntax has changed for Jasmine 2.0. The toHaveBeenCalled matcher will return true if the spy was called. The toHaveBeenCalledWith matcher will return true if the argument list matches any of the recorded calls to the spy.

 describe("A spy", function() {
  var foo, bar = null;

  beforeEach(function() {
    foo = {
      setBar: function(value) {
        bar = value;
      }
    };

    spyOn(foo, 'setBar');

    foo.setBar(123);
    foo.setBar(456, 'another param');
  });

  it("tracks that the spy was called", function() {
    expect(foo.setBar).toHaveBeenCalled();
  });

  it("tracks all the arguments of its calls", function() {
    expect(foo.setBar).toHaveBeenCalledWith(123);
    expect(foo.setBar).toHaveBeenCalledWith(456, 'another param');
  });

  it("stops all execution on a function", function() {
    expect(bar).toBeNull();
  });
});