Verifying function call and inspecting arguments using sinon spies

In your case, you are trying to see if bar was called, so you want to spy bar rather than foo.

As described in the doc :

function bar(x,y) {
  console.debug(x, y);
}
function foo(z) {
  bar(z, z+1);
}
// Spy on the function "bar" of the global object.
var spy = sinon.spy(window, "bar");

// Now, the "bar" function has been replaced by a "Spy" object
// (so this is not necessarily what you want to do) 

foo(1);

bar.getCall(0).args => should be [1,2]

Now, spying on the internals of the function strongly couples your test of "foo" to its implementation, so you'll fall into the usual "mockist vs classical" debate.

I agree with Adrian in saying that you probably wanted to spy on bar instead.

var barSpy = sinon.spy(bar);

Then to check that it was called once

assert(barSpy.calledOnce);

Just called at all

assert(barSpy.called)

Called x amount of times

assert.equal(barSpy.callCount, x);

If you want to extract the arguments from the first call of the spy:

var args = barSpy.getCalls()[0].args

Then you can do what you want with those arguments.

Shouldn't you be spying on bar, rather than foo?

var spy = sinon.spy(bar)

Call foo:

foo()

Check bar was called:

console.log(spy.calledOnce)