Chain mandatory and optional async operations

gistfile1.js

// Problem:
// - We have 2 operations that can and should run in parallel
// - Both operations may finish in arbitrary order
// - one operation is mandatory, the other is optional
// - the result promise is to be resolved with a merge of the intermediate ones, with optional overriding mandatory
// Question:
// Can this be done
// - in a more readable manner than the "result = new Promise..." code block
// - with less code than the "result = new Promise..." code block

// See http://jsfiddle.net/tkissing/4nqhcb59/ http://jsfiddle.net/tkissing/4nqhcb59/1/ http://jsfiddle.net/tkissing/4nqhcb59/2/

var mandatory = new Promise(function (resolve, reject) {
    function done() {
        resolve({
            a: 'mandatory', b: 'mandatory'
        });
        // or:
        reject('Sorry');
    }

    setTimeout(done, 100 * Math.random());
});
var optional = new Promise(function (resolve, reject) {
    function done() {
        resolve({
            b: 'optional'
        });
        // or:
        reject('Sorry');
    }

    setTimeout(done, 100 * Math.random());
});

var result = new Promise(function (resolve, reject) {
    return mandatory.then(function (obj) {
        optional.then(function (extras) {
            // optional worked => merge result into that of mandatory
            Object.keys(extras).forEach(function (prop) {
                obj[prop] = extras[prop];
            });
            resolve(obj);
        }, function () {
            // optional failed, just use result from mandatory
            resolve(obj);
        });
    }, reject);
});

var print = function (txt, err) {
    return function (obj) {
        (console[err ? 'warn' : 'log'])(txt, obj, Date.now());
    };
};

mandatory.then(print('mandatory'), print('mandatory', true));
optional.then(print('optional'), print('optional', true));
result.then(print('result'), print('result', true));