gistfile1.js

function foo(id) {
    return bar(id).then(function (obj) {
        if (!obj) {
            throw new Error('no obj');
        }
        return obj; // pass it through
    }).then(
        function (obj) {
            console.log('we got here, and have an obj; passing it through');
            return obj;
        },
        function (err) {
            console.log('we got an error; re-throwing it');
            throw err;
        }
    );
}