How to Fix CORS Errors in Your Frontend App Using a Proxy Server

Overview In this tutorial, you'll learn why HTTP requests or API calls made directly from the browser fail with a "Failed to Fetch" error, and how to fix it using the proxy pattern. Background If you've ever tried to make an HTTP request or API calls to an external server directly from a frontend JavaScript app, you've likely seen this error: It shows up in the browser console, but the request never goes through. It can feel confusing because the same call works fine in Postman or curl. The root cause is not your code. It is a browser security mechanism called CORS, and understanding it is the first step to fixing it. Same-Origin Policy Browsers enforce a rule called the Same-Origin Policy. It restricts web pages from making requests to a different origin than the one that served the page. Three things define an origin: Protocol — http or https Domain — localhost or example.com Port — 3000, 8080, etc. If any of these three differ between your frontend and the server you're calling, the