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

I've left an answer, if there is anything you would like for me to clarify please let me know. If the problem is resolve, indicate so by marking as answer. — EyuelDK, May 20 '17 at 05:28

EyuelDK · Accepted Answer · 2017-05-20T05:37:57.940

4

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>

edited May 20 '17 at 05:37

answered May 20 '17 at 05:23

EyuelDK

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thanks for the answer. btw, why not `window.foo` and why `window, "foo"` – harishr May 20 '17 at 05:39
1

the `spy` function needs to know the object and the method of that object to spy on. As I said in my answer, internally it is doing something like `object[method] = wrapped(object[method])` In javascript, you technically can't overwrite a function, but rather you redirect the reference of the function. The function referenced by `foo` is never changed, but the function referenced by the variable `foo` is changed, i.e. by the use of the object and method of the object. – EyuelDK May 20 '17 at 05:43
thanks a lot for great explanation.. that nailed it for me :) – harishr May 29 '17 at 04:14

how to spy on a function

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