Does Jasmine's spyOn() allow the spied on function to be executed? - unit-testing

Question

Does Jasmine's spyOn() method allow the spied on function to be executed, or does it, kind of - intercept the invocation when the spied method is (about to get) invoked, and returns true.
PS: Could anyone point me to the explanation of spyOn()'s inner workings?

Answer 1

Spy :
A spy can pretend to be a function or an object that you can use while writing unit test code to examine behavior of functions/objects
var Person = function() {};
Dictionary.prototype.FirstName = function() {
return "My FirstName";
};
Dictionary.prototype.LastName = function() {
return "My LastName";
};
Person.prototype.MyName = function() {
return FirstName() + " " + LastName();
};
Person.prototype.MyLocation = function() {
Return ”some location”;
};
describe("Person", function() {
it('uses First Name and Last Name for MyName', function() {
var person = new Person;
spyOn(person , "FirstName");
spyOn(person, "LastName");
person.MyName();
expect(person.FirstName).toHaveBeenCalled();
expect(person.LastName).toHaveBeenCalled();
});
});
Through SpyOn you can know whether some function has been / has not been called
expect(person. MyLocation).not.toHaveBeenCalled();
You can ensure that a spy always returns a given value and test it
spyOn(person, " MyName ").andReturn("My FirstNameMy LasttName ");
var result = person.MyName();
expect(result).toEqual("My FirstName My LasttName ");
Spies can call through to a fake function
it("can call a fake function", function() {
var fakeFun = function() {
alert("I am a spy!”);
return "hello";
};
var person = new person();
spyOn(person, "MyName").andCallFake(fakeFun);
person. MyName (); // alert
})
You can even create a NEW spy function or object and make use of it
it("can have a spy function", function() {
var person = new Person();
person.StreetAddress = jasmine.createSpy("Some Address");
person. StreetAddress ();
expect(person. StreetAddress).toHaveBeenCalled();
});

Answer 2

It just creates a mock(spy) object and injects it to your tested code.
It has three main purposes:
Testing your code if it calls spy: toHaveBeenCalled
Testing your code if it calls with appropriate parameters: toHaveBeenCalledWith
Testing your code for different return values from spy: and.callThrough