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