Asynchronous flow control

Asynchronous flow control is an unavoidable problem in javascript.
Synchronization is the execution of the next task after a task is executed. Asynchronous means that multiple tasks are executed at the same time. Therefore, if there is task B that requires the result of task A, it involves the problem of asynchronous flow control.

Callback Function
This is the most basic method of asynchronous programming

function f1(callback) {
        console.log('start to run f1.');
        setTimeout(function () {
            console.log('f1 is finished. callback f2');
            callback();
        },3000);
    }
    function f2() {
        console.log('start to run f2.');
    }
    f1(f2);

The running result is
start to run f1.
f1 is finished. callback f2
start to run f2.

Promise
Promise object is a new specification of ES6, each asynchronous task returns a Promise object, which has a then method, allowing to specify callback function

var getTestJson = function (url) {
        var promise = new Promise(function(resolve,reject){
            var xhr = new XMLHttpRequest();
            xhr.open("GET",url,true);
            xhr.onreadystatechange = function () {
                console.log("this.readyState:"+this.readyState+",this.status:"+this.status);
                if(this.readyState !== 4) {
                    return;
                }
                if(this.status === 200){
                    resolve(this.response);
                } else {
                    reject(new Error(this.statusText));
                }
            }
            xhr.responseType = "json";
            xhr.setRequestHeader("Accept","application/json");
            xhr.send();
        });
        return promise;
    };
    var url = "http://localhost:8000/getJson";
    getTestJson(url).then(function (json) {
        console.log('get the data back',json);
    },function (error) {
        console.log('error',error);
    });

getTestJson is an asynchronous task. After the task is executed, the .then method will be called
http://localhost:8000/getJson This address is the background that I use locally with nodejs