Take you step by step to understand asynchronous programming in JavaScript-JS Tutorial-php.cn

This article will introduce you to asynchronous programming in JavaScript. It has certain reference value. Friends in need can refer to it. I hope it will be helpful to everyone.

Take you step by step to understand asynchronous programming in JavaScript

Asynchronous means non-synchronous....

This section may be a bit boring, but it is a very important concept in JavaScript and is very useful. It is necessary to learn.

Purpose

Improve development efficiency and write easy-to-maintain code

Introduction question

Why does the page get stuck when requesting? ?

$.ajax({
  url: "www.xx.com/api",
  async: false, // true
  success: function(result) {
    console.log(result);
  },
});

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Why is the data updated but the DOM not updated? ?

// 异步批量更新DOM（vue-nextTick）
// <p id="app">{{num}}</p>
new Vue({
  el: "#app",
  data: {
    num: 0,
  },
  mounted() {
    let dom = document.getElementById("app");
    while (this.num !== 100) {
      this.num++;
    }
    console.log("Vue num=" + this.num, "DOM num=" + dom.innerHTML);
    // Vue num=100,DOM num=0
    // nextTick or setTimeout
  },
});

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The reasons for asynchronous generation

Cause: single thread (one point in time, only do one thing), the browser's JS engine is single threaded caused.

Single thread means that there is only one thread responsible for interpreting and executing IavaScript code in the JS engine. You might as well call it the main thread.

The so-called single thread means that only one task can be completed at a time. If there are multiple tasks, they must be queued. The previous task is completed before the next task is executed.

First take a look at the thread diagram of the browser kernel:

Take you step by step to understand asynchronous programming in JavaScript

Among them, The rendering thread and the JS thread are mutually exclusive .

Suppose there are two functions, one modifying and one deleting, operating a DOM node at the same time. If there are multiple threads, if the two threads are executed at the same time, there will definitely be a deadlock and there will be problems.

Why JS should be designed as single-threaded, because of the special environment of the browser.

Advantages and disadvantages of single thread:

The advantage of this mode is that it is relatively simple to implement and the execution environment is relatively simple; The disadvantage is that as long as one task takes a long time, the subsequent Tasks must be queued and waited, which will delay the execution of the entire program. Common browser unresponsiveness (suspended death) is often caused by a certain piece of Javascript code running for a long time (such as an infinite loop), causing the entire page to get stuck in this place and other tasks cannot be performed.

Common blockage (infinite loop):

while (true) {}

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JS was originally designed to be a script language that runs in the browser, so we didn’t want to make it so complicated, so we just designed It has become a single thread, that is, can only do one thing at a time.

In order to solve single-thread blockingthis shortcoming: asynchronous is generated.

Take the example of eating instant noodles:

Synchronous: Buy instant noodles => Boil water (staring) => Cook noodles => Eat instant noodles
Asynchronous : Buy instant noodles => Boil water (the water boils and the kettle sounds - callback) => Watch TV => Cook noodles (the noodles are ready and the kettle rings - callback) => Watch TV => Call me when it's done => Eat instant noodles

Watching TV is an asynchronous operation, and the sound of the kettle is a callback function.

Asynchronous Programming

Most of the code in JS is executed synchronously. Only a few functions are executed asynchronously. Asynchronously executed code requires asynchronous programming. .

Asynchronous code

setTimeout(() => {
  console.log("log2");
}, 0);
console.log("log1");
// ?? log1 log2

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Characteristics of asynchronous code: It is not executed immediately, but needs to wait and be executed at a certain point in the future.

Network request (Ajax)I/O operationTimer (setTimeout, setInterval) Rendering operationPromise(then)async/await

Synchronous code	Asynchronous code
##<script>`Code`

Callback function

The most common way to write asynchronous code is to use a callback function.

// 注意到click方法中是一个函数而不是一个变量
// 它就是回调函数
$("#btn_1").click(function() {
  alert("Btn 1 Clicked");
});
// 或者
function click() {
  // 它就是回调函数
  alert("Btn 1 Clicked");
}
$("#btn_1").click(click);

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The shortcomings of the callback function are also obvious, and it is easy to produce callback hell:

Take you step by step to understand asynchronous programming in JavaScript

Three ways of asynchronous programming

function getOneNews() {
  $.ajax({
    url: topicsUrl,
    success: function(res) {
      let id = res.data[0].id;
      $.ajax({
        url: topicOneUrl + id,
        success: function(ress) {
          console.log(ress);
          render(ress.data);
        },
      });
    },
  });
}

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function getOneNews() {
  axios
    .get(topicsUrl)
    .then(function(response) {
      let id = response.data.data[0].id;
      return axios.get(topicOneUrl + id);
    })
    .then((res) => {
      render(res.data.data);
    })
    .catch(function(error) {
      console.log(error);
    });
}

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async function getOneNews() {
  let listData = await axios.get(topicsUrl);
  let id = listData.data.data[0].id;
  let data = await axios.get(topicOneUrl + id);
  render(data.data.data);
}

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Online preview

Preview address: http://jsrun.net/s43Kp/embedded/all/ light

Question? ?

If multiple asynchronous codes exist at the same time, what should be the order of execution? Which one is executed first and which one is executed later?

Macro tasks and micro tasks

Division of asynchronous code, asynchronous code is divided into macro tasks and micro tasks.

##<script>setTimeout/setInterval

事件循环（Event loop）

Take you step by step to understand asynchronous programming in JavaScript

执行顺序：

执行整体代码<script>（宏任务）
执行所有微任务
执行一个宏任务
执行渲染线程
2->3->2->3...依次循环（在 2、3 步中又创建了新的宏、微任务）

重复从宏任务和微任务队列里拿出任务去执行。

总结

因为浏览器设计的原因，JS 线程和渲染线程互斥，所以 JS 线程被设计成了单线程。

因为单线程执行一些操作（如网络请求）时有堵塞的问题，所有产生了异步。

因为有了异步，所以产生了异步编程，从而有了回调函数。

因为回调函数写多了会产生回调地狱，所有又有了解决回调地狱的 Promise 写法

自 ES7 标准后有了比 Promise 更加优雅的写法 ———— async/await 写法，也是异步编程的最终解决方法。

因为 JS 的代码分为同步和异步代码，同步代码的执行顺序不必多说，自上而下的执行。

但是如果有多个异步的代码，他的执行顺序又是怎么的呢？？

为了解决多个异步代码的执行顺序问了，有了事件循环（EventLoop），将异步任务区分为宏任务、微任务，依据规则依次执行。

至此完！

练习

console.log("script start");
setTimeout(function() {
  console.log("timeout1");
}, 10);
new Promise((resolve) => {
  console.log("promise1");
  resolve();
  setTimeout(() => console.log("timeout2"), 10);
}).then(function() {
  console.log("then1");
});
console.log("script end");

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写出 log 的输出结果，并说出理由。

更多编程相关知识，请访问：编程视频！！

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source：segmentfault.com

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Macro tasks (not in a hurry)	Micro tasks (in a hurry)
Overall codePromise