How to spy a predefined object with sinon.js

Question

I'm trying to spy window.document with sinon.js. What I do is this:

var document = {
    getElementById = function() {}
}

sinon.spy(document, "getElementById").withArgs("foo").returnValues = ["bar"];

What I expect from this call is this: When document.getElementById is called with the argument "foo" the function must return "bar". What's my error?

If I define getElementById by myself like this I get the expected result:

document.getElementById = function(param) {
    if (param === "foo") return "bar";
}

Answer 1

You can only record calls on function and check that tey was called, but never change the behavior of the function. From the doc for withArgs:

Creates a spy that only records calls when the received arguments matche those passed to withArgs

Whar you are looking for is a sinon.stub:

sinon.stub(document, 'getElementById').withArgs('foo').returns(['bar'])