JavaScript Promise迷你书（中文版）--再学习

上次粗翻了一下，感觉没吃透，这次深入体会一下。

<script>
     function getURL(URL) {
         return new Promise(function(resolve, reject) {
             var req = new XMLHttpRequest();
             req.open('GET', URL, true);
             req.onload = function() {
                 if (req.status === 200) {
                     resolve(req.responseText);
                 } else {
                     reject(new Error(req.statusText));
                 }
             };
             req.onerror = function() {
                 reject(new Error(req.statusText));
             };
             req.send();
         });
    }

    var URL = "http://127.0.0.1:8888/test.html";
    getURL(URL).then(function onFulfilled(value) {
        console.log(value);
    }).catch(function onRejected(error) {
        console.error(error);
    });

    Promise.resolve(42).then(function(value) {
        console.log(value);
    });
</script>