Calling original function from Sinon.js Stub

You could use a closure. For example:

var obj = {
    foo: function () {
        console.log('foo');
    }
};

var stub = (function () {
    var originalFoo = obj.foo;
    return sinon.stub(obj, 'foo', function () {
        console.log('stub');
        originalFoo();
    });
}());

JSFiddle