How to intercept http requests with javascript

With the continuous development of network applications, Web front-end development is also changing with each passing day, and JavaScript, one of the essential languages ​​​​for Web front-end development, is also constantly developing. JavaScript can not only be used to achieve front-end dynamic effects, but can also be used to intercept HTTP requests.

What is intercepting HTTP requests?

HTTP, the full name is Hyper Text Transfer Protocol, which is the Hyper Text Transfer Protocol. In web applications, the communication between the client and the server is based on the HTTP protocol. In HTTP requests, we can intercept HTTP requests and modify or intercept the requests before they reach the server. In this way, we can modify or add HTTP request headers, or view request parameters and request results before the request is sent to the server.

Why intercept HTTP requests?

Intercepting HTTP requests allows us to better understand the overall running process and status of network applications. We can understand the real-time status of all requests and record them in real time, discover problems in time and handle them.

During the development process, we can modify, add or delete request headers by intercepting HTTP requests according to actual needs to achieve specific functions. For example, we can add some verification information to the request header, or when the front-end requests back-end data, we can add fixed parameters to the request, control the request process, etc.

How to intercept HTTP requests?

The browser provides some APIs that can be used to intercept HTTP requests, such as XMLHttpRequest and fetch. Let’s take a look at them one by one:

1. XMLHttpRequest interception request

In native JavaScript, we can use XMLHttpRequest to initiate a request. The readyState attribute representing the HTTP request has 5 states.

We can monitor and intercept HTTP requests for the readyState state.

let xhr = new XMLHttpRequest();

xhr.onreadystatechange = function() {
  if(xhr.readyState === 4) {  // see the below 5 states
    console.log( xhr.responseText );
  }
};

xhr.open("GET", "/test", true);

xhr.setRequestHeader("Content-Type", "application/json;charset=UTF-8");

xhr.send();

In the above code, we first create an XMLHttpRequest object xhr, and then monitor the status of xhr. When readyState is 4, it means that the request has been responded to.

2. fetch intercepts requests

fetch is also an API for network requests, which provides a simpler and more convenient request method.

fetch('/test')
  .then(function(response) {
    return response.json();
  })
  .then(function(myJson) {
    console.log(myJson);
  });

In fetch, we can use the then() function to react to the response. Here, the first function returns a response object response, and then we parse the response into JSON and put it in myJson.

We can also intercept and modify fetch requests to achieve specific functions.

Summary

Intercepting HTTP requests allows us to monitor and record the status of all requests in real time, and discover and deal with problems in a timely manner. At the same time, we can also intercept HTTP requests and modify the request headers to achieve specific functions. In web applications, intercepting HTTP requests is indispensable, and JavaScript is a common way to intercept HTTP requests. It can help us better understand the overall running status of the web application and implement specific functions.

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