Examples of promise

　　No nonsense, sticky Code

    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);
                    }
                }
            };
            the request.open (Method, URL); 
            request.send (Data); 
        }); 
    } 
    the let P = Ajax ( 'the GET', 'config.json' ); 
    p.then ( function (text) { // If successful AJAX , the content is obtained in response 
        the console.log (text); 
    }). the catch ( function (Status) { // If the AJAX fails to obtain a response code 
        the console.log (Status); 
    });

These are the realization of simulation ajax

    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")

These are used to test the execution order of promise