function doThingThatAcceptsCallback(arg1, callback) {
    doThingThatReturnsAPromise(arg1)
    .then(function resolved(result) {
        callback(null, result);
    })
    .catch(function rejected(error) {
        callback(error);
    });
}


// same with es6 functions
function doThingThatAcceptsCallbackES6(arg1, callback) {
    doThingThatReturnsAPromise(arg1)
    .then(result => callback(null, result))
    .catch(error => callback(error);
}