How Javascript sends HTTP requests

Javascript is a widely used scripting language that can be used in Web pages. It supports many functions, the most common of which is probably making requests over HTTP and receiving data and displaying it on a web page. In this article, we will cover how Javascript sends HTTP requests.

HTTP is a client-server protocol used to exchange data between web applications. For example, when you enter a URL into a web browser, the browser sends an HTTP request to the web server to obtain the HTML content of the page.

Javascript can use the XMLHttpRequest object to send HTTP requests. The XMLHttpRequest object allows you to exchange data with the server through JavaScript code without refreshing the page. The following are the basic steps for sending HTTP requests through the XMLHttpRequest object:

1. Create an XMLHttpRequest object

To send an HTTP request, you first need to create an XMLHttpRequest object. You can use the following code to create an XMLHttpRequest object:

var xhr = new XMLHttpRequest();

2. Open the HTTP request

The open() method of the XMLHttpRequest object is used to initialize the HTTP request parameters. The open() method accepts three parameters: the method of the HTTP request, the URL of the request, and a Boolean value indicating whether to execute the request asynchronously. The values ​​for these parameters are "GET", "http://example.com/api", and "true".

For example, the following code uses the GET method to request http://example.com/api:

xhr.open('GET', 'http://example.com/api', true);

3. Send HTTP request

The send() method of the XMLHttpRequest object uses For sending HTTP requests. This can be left blank if there is no data for the request. If you need to send data, please use it as a parameter of the send() method.

For example, the following code sends an HTTP request:

xhr.send();

4. Handling the server response

When the server responds to the XMLHttpRequest request, it calls an event handler. You can register an event handler on the XMLHttpRequest object to handle the server response. The following code demonstrates how to register an event handler:

xhr.onreadystatechange = function() {
  if (xhr.readyState === 4) {
    alert(xhr.responseText);
  }
};

In this event handler, we check the readyState property of the XMLHttpRequest object to determine whether the server has responded to the request. When the value of readyState is 4, it means that the server has responded to the request.

After making an HTTP request, we can check the statusCode and responseText properties of the XMLHttpRequest object to get the HTTP response received from the server. The statusCode attribute contains the status code of the response, such as 200 for success, and the responseText attribute contains the body content of the response.

In this article, we introduced how Javascript uses the XMLHttpRequest object to send HTTP requests. If you are developing a web application, it will be useful to know the methods of sending HTTP requests.

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