Imagine deploying a new Angular frontend with a Node.js/Express API backend, only to find that none of the API calls work once it’s live. Our team faced exactly that scenario – the app was effectively broken due to one missing HTTP response header. Everything worked fine locally, but in production, the browser console showed CORS errors because our API responses lacked the Access-Control-Allow-Origin header. That one “simple” header made the difference between a fully functional app and a seemingly broken frontend. In this article, we’ll explore how a misconfigured CORS policy can break an Angular application and how to diagnose and fix it on the server side (with a Node/Express example).

How CORS Issues Appear in Angular Applications

The Angular app on http://localhost:4200 requests data from an API on http://localhost:3000. The server responds without the required Access-Control-Allow-Origin header, so the browser’s CORS policy blocks the Angular app from seeing the response.