e2723 v2.js

/**
 * @param {Promise} promise1
 * @param {Promise} promise2
 * @return {Promise}
 */
var addTwoPromises = async function(promise1, promise2) {
    return promise1.then(val1 => promise2.then(val2 => val1 + val2));
};

/**
 * addTwoPromises(Promise.resolve(2), Promise.resolve(2))
 *   .then(console.log); // 4
 */