If Everything Is Running on Localhost, Why Do We Still Get CORS Errors? (opens in new tab)

One question almost every frontend developer asks at some point: "My frontend is running on localhost:3000 and my backend is running on localhost:5000. Both are on my own machine. Why am I still getting a CORS error?" It feels strange because both applications are literally running on the same laptop. The answer lies in how browsers define an Origin. What Is an Origin? An origin is made up of: Protocol + Domain + Port For example: and have: Same protocol ✅ Same domain ✅ Different ports ❌ Beca...