Nodejs handles option requests

When making an HTTP request, you may encounter a request called OPTIONS, which is a preflight request used to check whether the server allows the request before sending the actual request. Since this kind of request is not a regular request like GET or POST, sometimes there will be some problems in server-side processing. In this article, we will cover how to handle OPTIONS requests using Node.js.

What is an OPTIONS request?

OPTIONS request is an HTTP request used to check whether the server allows specific cross-domain requests. When using the XMLHttpRequest object or the Fetch API to send a cross-domain request, the browser will first send an OPTIONS request to check whether the server supports the request. If no response is received from the server, the browser refuses to send the request.

Steps in processing OPTIONS requests

Before processing OPTIONS requests, first understand the common methods of processing cross-domain requests. Commonly used methods include CORS (Cross-Origin Resource Sharing) and JSONP (JSON with padding). Next, we will use CORS to handle OPTIONS requests.

CORS is a standard HTTP request header used for cross-domain communication between browsers and servers. By setting CORS headers such as Access-Control-Allow-Origin and Access-Control-Allow-Methods, you can allow or restrict cross-domain access to specific domain names. Now, let’s introduce how to add CORS Headers to our Node.js application:

1. Install the express framework

First, you need to install the express framework. We can install it using the following command:

npm install express

2. Create Server

Next, we need to create a server. An Express server can be created and started using the following code:

const express = require('express');
const app = express();

app.listen(8080, () => {
    console.log(`Server is listening on port 8080`);
});

3. Handling OPTIONS requests

In express, you can use the use method to handle all types of routing. Before handling the OPTIONS request, let's create a basic route to test whether the server is running properly:

app.use('/', (req, res) => {
    res.send('Hello World!');
});

Now, we need to handle the OPTIONS request. The following is a basic example:

app.use('*', (req, res, next) => {
    res.header('Access-Control-Allow-Origin', '*');
    res.header('Access-Control-Allow-Methods', 'GET, POST, PUT, DELETE, OPTIONS');
    res.header('Access-Control-Allow-Headers', 'Content-Type, Authorization, Content-Length, X-Requested-With');

    // 跨域请求CORS前置验证时，会发出OPTIONS请求
    if (req.method === 'OPTIONS') {
        res.status(200).end();
    } else {
        next();
    }
});

Here we use the use method in Express to handle all routes. We set three CORS headers: Access-Control-Allow-Origin, Access-Control-Allow-Methods and Access-Control-Allow-Headers. When processing an OPTIONS request, the server must return a 200 OK response. Other HTTP requests must be passed to the next route handler using the next() method.

These are the basic steps for using Node.js to handle OPTIONS requests.

Conclusion

The OPTIONS request is a preflight request used to check whether the server allows specific cross-domain requests. Since it's not one of the regular requests, it has to be handled specially on the server side. In this article, we covered how to use CORS in Node.js to handle OPTIONS requests. When encountering an OPTIONS request, be sure to return a 200 OK response to allow the browser to process the actual request.

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