Client side config object for web apps

config.js

(function() {
    const fetchJSON = function(url) {
        return Q.Promise((resolve) => {
            const xhr = new XMLHttpRequest();
            xhr.open('GET', url);
            xhr.onreadystatechange = function() {
                if (xhr.readyState === 4 && xhr.status === 200) {
                    if (xhr.getResponseHeader('Content-Type') === 'application/json') {
                        resolve(JSON.parse(xhr.responseText));
                    }
                }
            };
            xhr.send(null);
        });
    };

    const extend = function(target) {
        for (let i = 1; i < arguments.length; ++i) {
            const original = arguments[i];
            if (typeof original !== 'object') continue;
            for (const j in original) {
                if (original.hasOwnProperty(j)) {
                    target[j] = typeof original[j] === 'object' ? extend({}, target[j], original[j]) : original[j];
                }
            }
        }
        return target;
    };

    fetchJSON('config/default.json')
        .then((default_config) => [default_config, fetchJSON('config/config.json')])
        .spread((default_config, env_config) => {
            window.config = extend({}, default_config, env_config);
        })
        ['catch']((err) => {
            console.log(err);
        })
    .done();
}());