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JS 教程

HTML CSS JAVASCRIPT SQL PYTHON JAVA PHP HOW TO W3.CSS C C++ C# BOOTSTRAP REACT MYSQL JQUERY EXCEL XML DJANGO NUMPY PANDAS NODEJS R TYPESCRIPT ANGULAR GIT POSTGRESQL MONGODB ASP AI GO KOTLIN SASS VUE DSA GEN AI SCIPY AWS CYBERSECURITY DATA SCIENCE

JS 版本

JS 版本 JS 2009 (ES5) JS 2015 (ES6) JS 2016 JS 2017 JS 2018 JS 2019 JS 2020 JS 2021 JS 2022 JS 2023 JS 2024 JS IE / Edge JS 历史

JS 对象

对象定义 对象原型 对象方法 对象属性 对象 Get / Set 对象保护

JS 函数

函数定义 函数参数 函数调用 函数 Call 函数 Apply 函数 Bind 函数闭包

JS 类

类入门 类继承 类静态

JS 异步

JS 回调 JS 异步 JS Promises JS Async/Await

JS HTML DOM

DOM 入门 DOM 方法 DOM Document DOM 元素 DOM HTML DOM 表单 DOM CSS DOM 动画 DOM 事件 DOM 事件监听器 DOM 导航 DOM 节点 DOM 集合 DOM 节点列表

JS Browser BOM

JS Window JS Screen JS Location JS History JS Navigator JS Popup Alert JS Timing JS Cookies

JS Web APIs

Web API 入门 Web 表单 API Web History API Web Storage API Web Worker API Web Fetch API Web Geolocation API

JS AJAX

AJAX 入门 AJAX XMLHttp AJAX Request AJAX Response AJAX XML 文件 AJAX PHP AJAX ASP AJAX 数据库 AJAX 应用 AJAX 示例

JS JSON

JSON 入门 JSON 语法 JSON vs XML JSON 数据类型 JSON 解析 JSON Stringify JSON 对象 JSON 数组 JSON 服务器 JSON PHP JSON HTML JSON JSONP

JS vs jQuery

jQuery 选择器 jQuery HTML jQuery CSS jQuery DOM

JS Graphics

JS Graphics JS Canvas JS Plotly JS Chart.js JS Google Chart JS D3.js

JS 示例

JS 示例 JS HTML DOM JS HTML 输入 JS HTML 对象 JS HTML 事件 JS 浏览器 JS 编辑器 JS 练习 JS 测验 JS 网站 JS 面试准备 JS Bootcamp JS 证书

JS 参考

JavaScript 对象 HTML DOM 对象


JavaScript Promises

"我保证会有一个结果!"

"Producing code" is code that can take some time

"Consuming code" is code that must wait for the result

A Promise is an Object that links Producing code and Consuming code

JavaScript Promise Object

A Promise contains both the producing code and calls to the consuming code

Promise Syntax

let myPromise = new Promise(function(myResolve, myReject) {
// "Producing Code" (May take some time)

  myResolve(); // when successful
  myReject();  // when error
});

// "Consuming Code" (Must wait for a fulfilled Promise)
myPromise.then(
  function(value) { /* 成功时的代码 */ },
  function(error) { /* 发生错误时的代码 */ }
);

When the producing code obtains the result, it should call one of the two callbacks

WhenCall
成功myResolve(result value)
ErrormyReject(error object)

Promise Object Properties

A JavaScript Promise object can be

  • Pending
  • Fulfilled
  • Rejected

The Promise object supports two properties: state and result.

While a Promise object is "pending" (working), the result is undefined.

When a Promise object is "fulfilled", the result is a value.

When a Promise object is "rejected", the result is an error object.

myPromise.statemyPromise.result
"pending"undefined
"fulfilled"a result value
"rejected"an error object

You cannot access the Promise properties state and result.

You must use a Promise method to handle promises.


Promise How To

Here is how to use a Promise

myPromise.then(
  function(value) { /* 成功时的代码 */ },
  function(error) { /* 发生错误时的代码 */ }
);

Promise.then() takes two arguments, a callback for success and another for failure.

Both are optional, so you can add a callback for success or failure only.

示例

function myDisplayer(some) {
  document.getElementById("demo").innerHTML = some;
}

let myPromise = new Promise(function(myResolve, myReject) {
  let x = 0;

// The producing code (this may take some time)

  if (x == 0) {
    myResolve("OK");
  } else {
    myReject("Error");
  }
});

myPromise.then(
  function(value) {myDisplayer(value);},
  function(error) {myDisplayer(error);}
);

自己动手试一试 »



JavaScript Promise Examples

To demonstrate the use of promises, we will use the callback examples from the previous chapter

  • 等待超时
  • Waiting for a File

等待超时

Example Using Callback

setTimeout(function() { myFunction("I love You !!!"); }, 3000);

function myFunction(value) {
  document.getElementById("demo").innerHTML = value;
}

自己动手试一试 »

Example Using Promise

let myPromise = new Promise(function(myResolve, myReject) {
  setTimeout(function() { myResolve("I love You !!"); }, 3000);
});

myPromise.then(function(value) {
  document.getElementById("demo").innerHTML = value;
});

自己动手试一试 »


Waiting for a file

Example using Callback

function getFile(myCallback) {
  let req = new XMLHttpRequest();
  req.open('GET', "mycar.html");
  req.onload = function() {
    if (req.status == 200) {
      myCallback(req.responseText);
    } else {
      myCallback("Error: " + req.status);
    }
  }
  req.send();
}

getFile(myDisplayer);

自己动手试一试 »

Example using Promise

let myPromise = new Promise(function(myResolve, myReject) {
  let req = new XMLHttpRequest();
  req.open('GET', "mycar.html");
  req.onload = function() {
    if (req.status == 200) {
      myResolve(req.response);
    } else {
      myReject("File not Found");
    }
  };
  req.send();
});

myPromise.then(
  function(value) {myDisplayer(value);},
  function(error) {myDisplayer(error);}
);

自己动手试一试 »


浏览器支持

ECMAScript 2015, also known as ES6, introduced the JavaScript Promise object.

The following table defines the first browser version with full support for Promise objects

Chrome 33 Edge 12 Firefox 29 Safari 7.1 Opera 20
Feb, 2014 2015 年 7 月 Apr, 2014 Sep, 2014 Mar, 2014


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