Promise-based JS script loader

script.js

function script(url) {

    if(Array.isArray(url)) {
        var self = this, prom = [];
        url.forEach(function(item) {
            prom.push(self.script(item));
        });
        return Promise.all(prom);
    }

    return new Promise(function (resolve, reject) {
        var r = false,
            t = document.getElementsByTagName("script")[0],
            s = document.createElement("script");

        s.type = "text/javascript";
        s.src = url;
        s.async = true;
        s.onload = s.onreadystatechange = function () {
            if (!r && (!this.readyState || this.readyState == "complete")) {
                r = true;
                resolve(this);
            }
        };
        s.onerror = s.onabort = reject;
        t.parentNode.insertBefore(s, t);
    });

}