Promise API extension to resolve them sequentially.

Promise.js

// Promise API extension to be able to resolve them sequentially.
Promise.serial = function (promises) {
    return promises.reduce((promiseChain, currentPromise) => {
        return promiseChain.then(chainResults =>
            // Using Promise.resolve() to be able to take any kind of 'promises' parameter: Promise, Function, Object, ...
            Promise.resolve().then(currentPromise).then(currentResult =>
                [ ...chainResults, currentResult ]
            )
        );
    }, Promise.resolve([]))
};