promise实例

　　不废话，粘代码

    function ajax(method, url, data) {let request = new XMLHttpRequest();return new Promise(function (resolve, reject) {request.onreadystatechange = function () {if (request.readyState === 4) {if (request.status === 200) {resolve(request.responseText);} else {reject(request.status);}}};request.open(method, url);request.send(data);});}let p = ajax('GET', 'config.json');p.then(function (text) { // 如果AJAX成功，获得响应内容
        console.log(text);}).catch(function (status) { // 如果AJAX失败，获得响应代码
        console.log(status);});

以上是模拟ajax实现

    let promise=new Promise((resolve, reject) => {console.log("1")reject("我错了");resolve("123");});promise.then(function(data){console.log("3")console.log(data)},function(err){console.log("4")console.log(err)})console.log("2")

以上是用来测试promise的执行顺序