JavaScript asynchronous programming 3 -- chain use of promise

charlee44 2021-05-03 18:26:46
javascript asynchronous programming chain use


In the last article 《JavaScript Asynchronous programming 2—— combination XMLHttpRequest Use Promise》 in , Briefly introduced Ajax And Promise Combined use of . such , We have two examples of asynchronous operations : Read a json file ; Loading images from an address . Think about it , If there are two asynchronous operations , They need to perform one operation before another ( For example, here , We store the image address in json In file , By visiting json To load the image ), How to do it ?

Detailed discussion

1️⃣ Back to hell

In order to realize the functions mentioned above , If we don't use Promise, Of course, it can also be realized by using callback function directly :

$(function () {
var url = "./1.json";
var req = new XMLHttpRequest();'GET', url);
req.onload = function () {
if (req.status == 200) {
var imgJson = JSON.parse(req.response);
var img = new Image();
img.onload = function () {
img.onerror = function () {
throw new Error("Load Image Error!");
img.src = imgJson[0];
} else {
throw new Error(req.statusText);
req.onerror = function () {
throw new Error("Network Error");

You can see here that we use two layers of nested callbacks , The asynchronous operation of loading images is in XMLHttpRequest Access request response callback , This allows access to json Access the image operation immediately after the request is finished . So let's take it a step further , You need to add a behavior , After loading the image, you can operate it ( For example, image processing )? In this case, we have to add another layer of nested callback functions . such , The program goes from top to bottom , The order from front to back will become from the outside to the inside —— The most intuitive inconvenience is ,"{}" There are more layers , Programs can become hard to read —— And this , It's called “ Back to hell ” 了 .

2️⃣Promise Realization

In order to solve “ Back to hell ” The problem of ,Promise emerge as the times require . As I said in previous articles ,Promise Purpose , I want asynchronous behavior to follow the same order as synchronous operation , To avoid nested callbacks . in other words , As long as the success of each implementation , That is to say then() In the method , Go back to the new Promise object , You can call the Promise Object's then() Method , In this way, asynchronous behavior can be like synchronous operation , Put them together in order . And the combination is chained , From front to back , This avoids multi-layer nesting :

$(function () {
function get(url) {
return new Promise(function (resolve, reject) {
var req = new XMLHttpRequest();'GET', url);
req.onload = function () {
// Even if it's 404 It will also enter the corresponding function , So we need to detect the State
if (req.status == 200) {
// Fulfill the promise , Returns the response text
} else {
// Done not done , Returns an error
// The corresponding function in case of an error
req.onerror = function () {
reject(Error("Network Error"));
// Send a request
function getImg(uri){
return new Promise(function(resolve, reject){
var img = new Image();
img.onload = function () {
img.onerror = function () {
reject(Error("Load Image Error!"));
img.src = uri;
var addressUri = "./1.json";
get(addressUri).then(function (response) {
var imgJson = JSON.parse(response);
return getImg(imgJson[0]);
}, function (error) {
console.error("Failed!", error);
}, function(error){
console.error("Failed!", error);

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