At current sinon version v2.3.1, you can use stub.resolves(value)
and stub.rejects(value)
function
For example, you can stub myClass.myFunction with following code
sinon.stub(myClass, 'myFunction').resolves('the value you want to return');
or
sinon.stub(myClass, 'myFunction').rejects('the error information you want to return');