Response to Preflight Request Doesn’t Pass Access Control Check – No ‘access-control-allow-origin’ Header

Response to preflight request doesn’t pass access control check – No ‘Access-Control-Allow-Origin’ header

If you have ever encountered the error message “Response to preflight request doesn’t pass access control check: No ‘Access-Control-Allow-Origin’ header”, you are not alone. This error occurs when making a cross-origin request from a web page using JavaScript, and the server does not include the necessary ‘Access-Control-Allow-Origin’ header in its response. In this blog post, we will explore the causes of this error and provide multiple solutions to resolve it.

Cause of the Error

The ‘Access-Control-Allow-Origin’ header is a security mechanism implemented by browsers to restrict cross-origin requests. When making a request from one domain to another, the browser sends an HTTP OPTIONS request (preflight request) to check if the server allows the cross-origin request. If the server does not respond with the appropriate ‘Access-Control-Allow-Origin’ header, the browser blocks the request, resulting in the error message.

Solution 1: Server-side Configuration

The first solution involves configuring the server to include the ‘Access-Control-Allow-Origin’ header in its response. This header specifies which domains are allowed to make cross-origin requests. To allow all domains, the server can include the following header:

Access-Control-Allow-Origin: *

If you want to restrict cross-origin requests to specific domains, you can replace the asterisk (*) with the desired domain(s) separated by commas. For example:

Access-Control-Allow-Origin: https://example.com, https://another-domain.com

Make sure to consult your server’s documentation or contact your server administrator to properly configure the ‘Access-Control-Allow-Origin’ header.

Solution 2: Proxy Server

If you do not have control over the server’s configuration or cannot modify it, you can use a proxy server to bypass the cross-origin restriction. A proxy server acts as an intermediary between the client and the server, forwarding requests and responses.

To set up a proxy server, you can use a tool like http-proxy-middleware in Node.js. Here’s an example of how to configure the proxy server:

const express = require('express');
const { createProxyMiddleware } = require('http-proxy-middleware');

const app = express();

app.use('/api', createProxyMiddleware({ target: 'https://api.example.com', changeOrigin: true }));

app.listen(3000, () => {
  console.log('Proxy server listening on port 3000');
});

In this example, any request made to ‘/api’ on the proxy server will be forwarded to ‘https://api.example.com’. The ‘changeOrigin’ option ensures that the ‘Access-Control-Allow-Origin’ header is included in the response from the target server.

Solution 3: JSONP (JSON with Padding)

If the server does not support the ‘Access-Control-Allow-Origin’ header, another solution is to use JSONP (JSON with Padding). JSONP is a technique that allows making cross-origin requests by dynamically adding a script tag to the HTML page.

Here’s an example of how to use JSONP:

In this example, we create a script tag dynamically and set its source to the desired API endpoint. We also include a ‘callback’ query parameter with the name of the function that will handle the response. The server should wrap the response data in a function call matching the provided callback name.

Conclusion

The “Response to preflight request doesn’t pass access control check: No ‘Access-Control-Allow-Origin’ header” error can be frustrating, but with the solutions provided in this blog post, you should be able to overcome it. Whether it’s configuring the server, using a proxy server, or employing JSONP, you now have multiple options to resolve this issue and continue building your JavaScript applications.

Remember to choose the solution that best fits your specific requirements and constraints. Happy coding!