Difference between fake, spy, stub and mock of sinon library ( sinon fake vs spy vs stub vs mock )

Question

I tried to understand difference between sinon library's fake, spy, stub and mock but not able to understand it clearly.

Can anybody help me to understand about it?

Answer 1

Just for understanding purpose call:

FuncInfoCollector = is a Function that records arguments, return value, the value of this(context) and exception thrown (if any) for all of its calls. (this FuncInfoCollector is dummy name given by me, it is not present in SINON lib)

---

Fake = FuncInfoCollector + can only create a fake function. To replace a function that already exists in the system under test you call sinon.replace(target, fieldname, fake). You can wrap an existing function like this:

const org = foo.someMethod;
sinon.fake((...args) => org(...args));

A fake is immutable: once created, the behavior can't be changed.

var fakeFunc = sinon.fake.returns('foo');
fakeFunc();

// have call count of fakeFunc ( It will show 1 here)
fakeFunc.callCount;   

Spy = FuncInfoCollector + can create new function + It can wrap a function that already exists in the system under test.

Spy is a good choice whenever the goal of a test is to verify something happened.

// Can be passed as a callback to async func to verify whether callback is called or not?
const spyFunc = sinon.spy();

// Creates spy for ajax method of jQuery lib
sinon.spy(jQuery, "ajax");       

// will tell whether jQuery.ajax method called exactly once or not 
jQuery.ajax.calledOnce 

Stub = spy + it stubs the original function ( can be used to change behaviour of original function).

var err = new Error('Ajax Error');

// So whenever jQuery.ajax method is called in a code it throws this Error
sinon.stub(jQuery, "ajax").throws(err) 

// Here we are writing assert to check where jQuery.ajax is throwing an Error or not
sinon.assert.threw(jQuery.ajax(), err);

Mock = Stub + pre-programmed expectations.

var mk = sinon.mock(jQuery)

// Should be called atleast 2 time and almost 5 times
mk.expects("ajax").atLeast(2).atMost(5); 

// It throws the following exception when called ( assert used above is not needed now )
mk.expects("ajax").throws(new Error('Ajax Error')) 

// will check whether all above expectations are met or not, hence assertions aren't needed
mk.verify(); 

Please have a look at this link also sinon.replace vs sinon.stub just to replace return value?

Answer 2

Just to add some more info to the otherwise good answer, we added the Fake API to Sinon because of shortcomings of the other original APIs (Stub and Spy). The fact that these APIs are chainable led to constant design issues and recurring user-problems and they were bloated to cater to quite unimportant use cases, which is why we opted for creating a new immutable API that would be simpler to use, less ambiguous and cheaper to maintain. It was built on top of the Spy and Stub Apis to let Fakes be somewhat recognizable and have explicit method for replacing props on objects (sinon.replace(obj,'prop',fake)).

Fakes can essentially be used anywhere a stub or spy can be used and so I have not used the old APIs myself in 3-4 years, as code using the more limited fakes is simpler to understand for other people.