试验 AJAX 和 API：初学者综合指南

1.AJAX和API简介

什么是 AJAX？

AJAX 代表异步 JavaScript 和 XML。这是一种用于 Web 开发的技术，用于创建动态和交互式网页，而无需重新加载整个页面。使用 AJAX，您可以从服务器请求数据并异步更新网页的部分内容，这意味着用户可以在后台处理请求时继续与页面交互。

什么是 API？

API（应用程序编程接口）是一组允许不同软件应用程序相互通信的规则和定义。在 Web 开发环境中，API 通常用于与 Web 服务器交互、检索数据和发送数据。 API 可以是公共的（任何人都可以使用）或私有的（仅限特定用户或应用程序）。

为什么同时使用 AJAX 和 API？

当您同时使用 AJAX 和 API 时，您可以使 Web 应用程序动态地获取、发送和更新数据。这种组合使您可以构建丰富、响应灵敏且交互式的 Web 应用程序，从而提供流畅的用户体验，而无需不断重新加载页面。

AJAX 和 API 的实际应用

• 社交媒体源：Twitter 和 Facebook 等平台使用 AJAX 动态加载新帖子和评论，而无需重新加载页面。
• 天气应用程序：天气应用程序从 API 获取实时天气数据以显示当前状况和预报。
• 电子商务网站：在线商店使用 AJAX 来实现过滤产品、更新购物车以及无需重新加载页面即可处理订单等功能。
• 地图和位置服务：Google 地图等服务使用 AJAX 根据用户输入动态加载地图图块和位置数据。

2.AJAX 入门

了解异步 JavaScript 和 XML (AJAX)

AJAX 不是单一技术，而是多种技术协同工作的组合：

• JavaScript：用于处理用户交互并发出异步请求。
• XML/JSON：客户端和服务器之间交换数据的格式。 JSON（JavaScript 对象表示法）如今更常用。
• HTML/CSS：用于构建网页内容并设置样式。
• DOM（文档对象模型）：表示网页的结构并允许 JavaScript 与其交互。

AJAX 的基本概念

1. 异步通信：AJAX 允许网页从服务器异步发送和接收数据，这意味着浏览器无需等待服务器的响应即可继续执行其他任务。
2. 部分页面更新：AJAX 无需重新加载整个页面，而是可以更新网页的特定部分，从而提高性能和用户体验。
3. 客户端-服务器通信：AJAX 使客户端 JavaScript 能够与服务器端 API 进行通信以获取或发送数据。

AJAX 的工作原理

1. 用户操作：用户触发事件，例如单击按钮或在表单中输入文本。
2. JavaScript 执行：JavaScript 捕获事件并创建 XMLHttpRequest 对象。
3. 服务器请求：XMLHttpRequest 对象向服务器发送请求。
4. 服务器处理：服务器处理请求并将响应发送回客户端。
5. 客户端更新：JavaScript 接收服务器响应并动态更新网页。

第一个使用 JavaScript 的 AJAX 请求

为了演示 AJAX 的基础知识，让我们创建一个简单的 HTML 页面并使用 JavaScript 向服务器发送 AJAX 请求。

分步指南：

1. 创建 HTML 文件：

49099650ebdc5f3125501fa170048923   93f0f5c25f18dab9d176bd4f6de5d30e  
    1fc2df4564f5324148703df3b6ed50c1  
    4f2fb0231f24e8aef524fc9bf9b9874f  
    b2386ffb911b14667cb8f0f91ea547a7AJAX Example6e916e0f7d1e588d4f442bf645aedb2f   9c3bca370b5104690d9ef395f2c5f8d1   6c04bd5ca3fcae76e30b72ad730ca86d  
    4a249f0d628e2318394fd9b75b4636b1AJAX Request Example473f0a7621bec819994bb5020d29372a  
    f86b0e4b92265ac35a2a70e8fe37df94Load Data65281c5ac262bf6d81768915a4a77ac0  
    d41cbdeca95710cb1424c563591ee03616b28748ea4df4d9c2150843fecfba68  
    32d8a8489ccf6ad22016e69b0555932f2cacc6d41bbb37262a98f745aa00fbf0   36cc49f0c466276486e50c850b7e4956   73a6ac4ed44ffec12cee46588e518a5e

2.创建 JavaScript 文件 (app.js):

    document.getElementById('loadData').addEventListener('click', function() {  
        const xhr = new XMLHttpRequest();  
        xhr.open('GET', 'https://jsonplaceholder.typicode.com/posts', true);  

        xhr.onload = function() {  
            if (xhr.status === 200) {  
                document.getElementById('dataContainer').innerHTML = xhr.responseText;  
            } else {  
                console.error('Failed to load data');  
            }  
        };  

        xhr.send();  
    });

3.测试AJAX请求:

• Open the HTML file in a web browser.
• Click the “Load Data” button to trigger the AJAX request.
• Observe the fetched data displayed in the dataContainer div

3. Understanding APIs

Definition of APIs

An API (Application Programming Interface) is a set of rules and protocols that allows different software applications to communicate with each other. APIs define the methods and data formats that applications can use to interact with each other, making it easier to build software by providing reusable components and services.

Types of APIs

1. REST (Representational State Transfer): REST APIs are the most common type of API used in web development. They are based on standard HTTP methods like GET, POST, PUT, and DELETE and are stateless, meaning each request is independent of others.
2. SOAP (Simple Object Access Protocol): SOAP is a protocol for exchanging structured information in web services. It uses XML for message formatting and relies on application layer protocols like HTTP and SMTP.
3. GraphQL: A newer API standard that allows clients to request exactly the data they need. Unlike REST, GraphQL uses a single endpoint and supports complex queries and mutations.

API Endpoints and Methods

• Endpoint: An endpoint is a specific URL where an API is accessed. It represents a specific function or resource within the API.
• HTTP Methods: The method used to interact with an endpoint. Common methods include:
• GET: Retrieve data from the server.
• POST: Send data to the server to create a new resource.
• PUT: Update an existing resource on the server.
• DELETE: Remove a resource from the server.

Working with JSON and XML Formats

APIs typically use JSON (JavaScript Object Notation) or XML (eXtensible Markup Language) to format data. JSON is more lightweight and easier to read, making it the preferred choice for most modern APIs.

Example JSON Response:

{  
   "id": 1,  
   "title": "Example Post",  
   "body": "This is an example of a post.",  
   "userId": 1  
}

Example XML Response:

ec655f8ae13ea154ec5dc6589cbe502a  
   53384f78b45ee9f1e3082cf378b9c5b4142c6f2d00583d752e4637b73e8eb9308  
   b2386ffb911b14667cb8f0f91ea547a7Example Post6e916e0f7d1e588d4f442bf645aedb2f  
   6c04bd5ca3fcae76e30b72ad730ca86dThis is an example of a post.36cc49f0c466276486e50c850b7e4956  
   0264c0739a3aa848263f180570ab986917a264015ae2ed1b3a0f49f37420e0cd8  
0cf29df1c39b9602336061a989a606e3

4. Making Your First AJAX Request

Setting Up a Basic HTML and JavaScript Environment

To make your first AJAX request, you need a basic HTML and JavaScript environment. Follow these steps:

1. Create an HTML File:

    8b05045a5be5764f313ed5b9168a17e6  
    49099650ebdc5f3125501fa170048923  
    93f0f5c25f18dab9d176bd4f6de5d30e  
        1fc2df4564f5324148703df3b6ed50c1  
        4f2fb0231f24e8aef524fc9bf9b9874f  
        b2386ffb911b14667cb8f0f91ea547a7First AJAX Request6e916e0f7d1e588d4f442bf645aedb2f  
    9c3bca370b5104690d9ef395f2c5f8d1  
    6c04bd5ca3fcae76e30b72ad730ca86d  
        4a249f0d628e2318394fd9b75b4636b1Fetch Data with AJAX473f0a7621bec819994bb5020d29372a  
        59abb0b237ca04f77b84b0a984fa204fFetch Data65281c5ac262bf6d81768915a4a77ac0  
        c6d0fc20cfda62e61a924a57f5913b8f16b28748ea4df4d9c2150843fecfba68  
        166bab32b60e87199ea5f9ef274384382cacc6d41bbb37262a98f745aa00fbf0  
    36cc49f0c466276486e50c850b7e4956  
    73a6ac4ed44ffec12cee46588e518a5e

Create a JavaScript File (ajax.js):

   document.getElementById('fetchDataBtn').addEventListener('click', function() {  
        const xhr = new XMLHttpRequest();  
        xhr.open('GET', 'https://jsonplaceholder.typicode.com/posts/1', true);  

        xhr.onload = function() {  
            if (xhr.status === 200) {  
                const data = JSON.parse(xhr.responseText);  
                document.getElementById('dataDisplay').innerHTML = `  
                    c1a436a314ed609750bd7c7d319db4da${data.title}2e9b454fa8428549ca2e64dfac4625cd  
                    e388a4556c0f65e1904146cc1a846bee${data.body}94b3e26ee717c64999d7867364b1b4a3  
                `;  
            } else {  
                console.error('Error fetching data');  
            }  
        };  

        xhr.onerror = function() {  
            console.error('Request failed');  
        };  

        xhr.send();  
    });

3.Test the AJAX Request:

• Open the HTML file in a web browser.
• Click the “Fetch Data” button to trigger the AJAX request.
• Observe the fetched data displayed in the dataDisplay div.

Sending a GET Request Using XMLHttpRequest

The XMLHttpRequest object is used to interact with servers. It allows you to make HTTP requests to retrieve or send data without reloading the page.

Steps to Send a GET Request:

1. Create an XMLHttpRequest Object: const xhr = new XMLHttpRequest();
2. Open a Connection: xhr.open('GET', 'https://jsonplaceholder.typicode.com/posts/1', true);
3. Define a Callback Function: xhr.onload = function() { /* Handle response */ };
4. Send the Request: xhr.send();

Handling Server Responses

When the server responds to an AJAX request, the response is available in the xhr.responseText property. You can use JavaScript to process this data and update the webpage dynamically.

Debugging AJAX Requests

To debug AJAX requests, use browser developer tools:

• Network Tab: Monitor HTTP requests and responses.
• Console Tab: Log errors and messages for debugging.

5. Using Fetch API for AJAX Requests

Introduction to Fetch API

The Fetch API is a modern alternative to XMLHttpRequest for making HTTP requests. It provides a more powerful and flexible feature set and returns Promises, making it easier to handle asynchronous operations.

Making GET and POST Requests with Fetch

Example of a GET Request with Fetch:

fetch('https://jsonplaceholder.typicode.com/posts/1')<br><br>
    .then(response => response.json())<br><br>
    .then(data => {<br><br>
        document.getElementById('dataDisplay').innerHTML = <br>
            &lt;h2&gt;${data.title}&lt;/h2&gt;  <br>
            &lt;p&gt;${data.body}&lt;/p&gt;  <br>
;<br><br>
    })<br><br>
    .catch(error => console.error('Error fetching data:', error));<br>

Example of a POST Request with Fetch:

fetch('https://jsonplaceholder.typicode.com/posts', {<br><br>
    method: 'POST',<br><br>
    headers: {<br><br>
        'Content-Type': 'application/json'<br><br>
    },<br><br>
    body: JSON.stringify({<br><br>
        title: 'New Post',<br><br>
        body: 'This is a new post.',<br><br>
        userId: 1<br><br>
    })<br><br>
})<br><br>
.then(response => response.json())<br><br>
.then(data => console.log('Post created:', data))<br><br>
.catch(error => console.error('Error creating post:', error));<br>

Handling JSON Responses

The Fetch API provides a json() method to parse the response body as JSON. This method returns a Promise that resolves with the parsed JSON data.

Error Handling with Fetch

Use .catch() to handle errors in Fetch requests. This method catches any errors that occur during the fetch operation or while processing the response.

6. Interacting with RESTful APIs

What is REST?

REST (Representational State Transfer) is an architectural style for designing networked applications. RESTful APIs follow specific conventions for managing resources over the web.

RESTful API Conventions

• Stateless: Each request from a client to a server must contain all the information needed to understand and process the request.
• Resource-Based: Resources (such as users, posts, or products) are identified by URLs.
• Standard Methods: RESTful APIs use standard HTTP methods (GET, POST, PUT, DELETE) to perform operations on resources.

Sending Requests to RESTful APIs

Example of Sending a GET Request to a RESTful API:

fetch('https://jsonplaceholder.typicode.com/posts')<br><br>
    .then(response => response.json())<br><br>
    .then(posts => {<br><br>
        posts.forEach(post => {<br><br>
            console.log(post.title);<br><br>
        });<br><br>
    })<br><br>
    .catch(error => console.error('Error fetching posts:', error));<br>

CRUD Operations with AJAX

CRUD stands for Create, Read, Update, Delete — four fundamental operations for managing data.

• Create: Use POST to add new data.
• Read: Use GET to retrieve data.
• Update: Use PUT or PATCH to modify existing data.
• Delete: Use DELETE to remove data.

Example of CRUD Operations with Fetch:

CRUD Operations with AJAX

CRUD stands for Create, Read, Update, Delete—four fundamental operations for managing data.

Create: Use POST to add new data.

Read: Use GET to retrieve data.

Update: Use PUT or PATCH to modify existing data.

Delete: Use DELETE to remove data.

Example of CRUD Operations with Fetch:

7. Advanced AJAX Techniques

Handling CORS (Cross-Origin Resource Sharing)

CORS (Cross-Origin Resource Sharing) is a security feature that restricts web pages from making requests to a different domain than the one that served the web page. To work with APIs across different domains, the server must enable CORS.

Using Promises for Better AJAX Management

Promises simplify the management of asynchronous operations in JavaScript. They provide a more readable way to handle AJAX requests compared to callbacks.

Example of Promises with Fetch:

fetch('/api/posts')<br><br>
    .then(response => response.json())<br><br>
    .then(data => console.log(data))<br><br>
    .catch(error => console.error('Error:', error));<br>

Working with Async/Await for Cleaner Code

Async/Await is syntactic sugar built on top of Promises, making asynchronous code easier to read and write.

Example of Async/Await:

async function fetchPosts() {<br><br>
    try {<br><br>
        const response = await fetch('/api/posts');<br><br>
        const data = await response.json();<br><br>
        console.log(data);<br><br>
    } catch (error) {<br><br>
        console.error('Error:', error);<br><br>
    }<br><br>
}  

<p>fetchPosts();<br>
</p>

Chaining Multiple AJAX Requests

To handle multiple AJAX requests in sequence, use Promises or Async/Await to chain requests.

Example of Chaining AJAX Requests:

fetch('/api/user')<br><br>
    .then(response => response.json())<br><br>
    .then(user => fetch(/api/posts?userId=${user.id}))<br><br>
    .then(response => response.json())<br><br>
    .then(posts => console.log(posts))<br><br>
    .catch(error => console.error('Error:', error));<br>

1. Building a Real-World Application with AJAX and APIs

Setting Up a Sample Project

To build a real-world application, you need a backend API and a frontend interface. For this example, we’ll use a simple API to fetch weather data.

Integrating a Third-Party API (e.g., OpenWeatherMap)

1. Sign Up for an API Key: Register for an API key on the OpenWeatherMap website.
2. Fetch Weather Data: Use AJAX to fetch weather data based on the user’s input.

Example of Fetching Weather Data:

const apiKey = 'YOUR_API_KEY';<br><br>
const city = 'London';<br><br>
fetch(https://api.openweathermap.org/data/2.5/weather?q=${city}&amp;appid=${apiKey})<br><br>
    .then(response => response.json())<br><br>
    .then(data => {<br><br>
        document.getElementById('weatherDisplay').innerHTML = <br>
            &lt;h2&gt;Weather in ${data.name}&lt;/h2&gt;  <br>
            &lt;p&gt;Temperature: ${(data.main.temp - 273.15).toFixed(2)}°C&lt;/p&gt;  <br>
            &lt;p&gt;Condition: ${data.weather[0].description}&lt;/p&gt;  <br>
;<br><br>
    })<br><br>
    .catch(error => console.error('Error fetching weather data:', error));<br>

Creating Dynamic and Interactive Elements with AJAX

Use JavaScript to create dynamic elements that update based on user input or server responses.

Example of Creating Dynamic Elements:

document.getElementById('searchButton').addEventListener('click', function() {<br><br>
    const city = document.getElementById('cityInput').value;<br><br>
    fetchWeatherData(city);<br><br>
});  

<p>function fetchWeatherData(city) {<br><br>
    fetch(https://api.openweathermap.org/data/2.5/weather?q=${city}&amp;appid=${apiKey})<br><br>
        .then(response => response.json())<br><br>
        .then(data => {<br><br>
            document.getElementById('weatherDisplay').innerHTML = <br>
                &lt;h2&gt;Weather in ${data.name}&lt;/h2&gt;  <br>
                &lt;p&gt;Temperature: ${(data.main.temp - 273.15).toFixed(2)}°C&lt;/p&gt;  <br>
                &lt;p&gt;Condition: ${data.weather[0].description}&lt;/p&gt;  <br>
;<br><br>
        })<br><br>
        .catch(error => console.error('Error fetching weather data:', error));<br><br>
}<br>
</p>

Displaying Data from API Responses

Use HTML and CSS to format the data received from APIs. JavaScript allows you to manipulate the DOM and display the data dynamically.

9. Optimizing AJAX Requests

Debouncing and Throttling API Calls

Debouncing and Throttling are techniques used to limit the rate at which a function is executed. This is especially useful when working with APIs to avoid unnecessary requests.

Example of Debouncing:

function debounce(func, delay) {<br><br>
    let timeout;<br><br>
    return function(...args) {<br><br>
        clearTimeout(timeout);<br><br>
        timeout = setTimeout(() => func.apply(this, args), delay);<br><br>
    };<br><br>
}  

<p>const fetchWeatherDebounced = debounce(fetchWeatherData, 300);<br><br>
document.getElementById('cityInput').addEventListener('input', function() {<br><br>
    fetchWeatherDebounced(this.value);<br><br>
});<br>
</p>

Managing Request States and Loading Indicators

Use JavaScript to manage the state of your AJAX requests and provide feedback to users, such as loading indicators or error messages.

Example of Managing Request States:

function fetchWeatherData(city) {<br><br>
    const loader = document.getElementById('loader');<br><br>
    loader.style.display = 'block'; // Show loader  
<div class="highlight js-code-highlight">
<pre class="highlight plaintext">fetch(`https://api.openweathermap.org/data/2.5/weather?q=${city}&amp;appid=${apiKey}`)  
    .then(response =&gt; response.json())  
    .then(data =&gt; {  
        loader.style.display = 'none'; // Hide loader  
        // Display weather data...  
    })  
    .catch(error =&gt; {  
        loader.style.display = 'none'; // Hide loader  
        console.error('Error fetching weather data:', error);  
    });  

}