With the rapid development of Internet technology, web applications have become one of the most common forms of the Internet today. JavaScript, as a core technology in the front-end of web pages, is becoming more and more important. Among them, the rise of AJAX technology allows web applications to interact with backend servers for data, and the page can be dynamically updated without completely refreshing the page. However, in some special cases, in order to meet the needs of the application, we need to customize some specific request headers. At this time, JavaScript custom request headers become particularly important.

1. What is a request header?

In a web application, when the browser sends a request to the server, the request will contain some additional information about the request, which is called a request header and usually contains the following content:

• Accept: Data type supported by the browser. For example: Accept: text/html,application/xhtml xml,application/xml;q=0.9,image/webp,/;q=0.8
• Referer: The source address of the requested page. For example: Referer: http://www.example.com/index.html
• User-Agent: Client software information that initiated the request. For example: User-Agent: Mozilla/5.0 (Windows NT 10.0; WOW64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/87.0.4280.141 Safari/537.36

2. Why you need to customize the request header ?

Sometimes, web applications require some special request headers to meet some specific needs, such as:

1. Cross-domain requests

In cross-domain requests When making a request, the server can control the allowed request headers through the Access-Control-Allow-Headers parameter. If you need to add some special parameters to the request header, you need to customize the request header through JavaScript.

2. IP restrictions

Some servers will restrict based on the IP address in the request header. If we want to send requests through different IP addresses, we need to customize the request. head.

3. Settings for loading resources

Some resources require specific settings, such as resource extraction, which need to be specified through the request header.

3. How to customize request headers?

In JavaScript, you can customize the request header through the xhr.setRequestHeader() method. The specific code is as follows:

let xhr = new XMLHttpRequest();
xhr.open('POST', 'http://www.example.com/api', true);
xhr.setRequestHeader('Content-Type', 'application/json');
xhr.onload = function () {
    console.log('请求成功');
};
xhr.send(JSON.stringify({name: 'Alice', age: 18}));

In the above code, we added the request header Content-Type: application/json through the xhr.setRequestHeader() method, and sent a JSON format data through the send() method { name: 'Alice', age: 18}.

In some cases, we need to add multiple parameters, which can be added through traversal. The sample code is as follows:

let data = {
    name: 'Alice',
    age: 18
};
let xhr = new XMLHttpRequest();
xhr.open('POST', 'http://www.example.com/api', true);
for (let key in data) {
    if (data.hasOwnProperty(key)) {
        xhr.setRequestHeader(key, data[key]);
    }
}
xhr.onload = function () {
    console.log('请求成功');
};
xhr.send();

4. Summary

By customizing request headers, we can meet some specific needs and realize more functions of web applications. However, when setting custom request headers, we need to be careful not to add unnecessary header information to avoid unnecessary trouble.

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