promise应用于ajax

promise应用于ajax，可以在本页打开控制台，复制代码试验

var url = 'https://www.cnblogs.com/mvc/blog/news.aspx?blogApp=dkplus';
var url1 = 'https://www.cnblogs.com/mvc/blog/calendar.aspx?blogApp=dkplus&dateStr=';

function getJSON(url) {
    return new Promise(function(resolve, reject) {
        var XHR = new XMLHttpRequest();
        XHR.open('GET', url, true);
        XHR.send();

        XHR.onreadystatechange = function() {
            if (XHR.readyState == 4) {
                if (XHR.status == 200) {
                    try {
                        var response = XHR.responseText;
                        resolve(response);
                    } catch (e) {
                        reject(e);
                    }
                } else {
                    reject(new Error(XHR.statusText));
                }
            }
        }
    })
}

function renderAll() {
    return Promise.all([getJSON(url), getJSON(url1)]);
}

// 全部成功
renderAll().then(function(value) {
    console.log(value);
})

function renderRace() {
    return Promise.race([getJSON(url), getJSON(url1)]);
}

// 有数据返回的时候
renderRace().then(function(value) {
    console.log(value);
})