how to spy on a function

Question

I am trying to spy on a global function e.g.

function foo() {

}

but the below test is failing, how to do that

    var spy = sinon.spy(foo);
    foo();
    expect(spy.callCount).to.equal(1);

** EDIT **

If I do it like below then it works

    var spy = sinon.spy(window, "foo");
    foo();
    expect(spy.callCount).to.equal(1);

So whats the difference

Answer 1

Using var spy = sinon.spy(foo); doesn't wrap the function foo as you might think. It actually returns a spy that contains the wrapped function.

Using var spy = sinon.spy(window, "foo"); actually wraps the foo method. Internally, window["foo"] = <wrapped function> is being done, thus replacing your the function referenced by foo to the wrapped function.

For var spy = sinon.spy(foo); to work you must not call foo() instead you need to call spy()

function foo() {

}

console.log('BEGIN sinon.spy(foo)');
var spy = sinon.spy(foo);
spy();
foo(); // called foo, but no effect on callCount
foo(); // called foo, but no effect on callCount
foo(); // called foo, but no effect on callCount
console.log('Call Count:', spy.callCount);

console.log('BEGIN sinon.spy(window, \'foo\')');
var spy = sinon.spy(window, 'foo');
spy();
foo(); 
foo(); 
foo();
console.log('Call Count:', spy.callCount);

<script src="https://cdnjs.cloudflare.com/ajax/libs/sinon.js/1.15.4/sinon.min.js"></script>