Promise-based cache

index.js

/**
 * @param {String} url
 * 
 * @return {Promise}
 */
function httpGet(url) {/*...*/}

var cache = {};
module.exports = function (url) {
    // if cacne -> return
    // else request and cache on resolve
    return cache[url] || httpGet(url).then(function (body) {
        // cache and forward result
        return cache[url] = promise, body;
    });
};