CORS error: Access-Control-Allow-Origin

CORS ERROR in the browser is frequently misdiagnosed as a JavaScript bug when the root cause is a network request failure, or vice versa. The browser's DevTools console and Network panel together provide the full picture. When a fetch() or XMLHttpRequest fails, the JavaScript error thrown describes the network failure, not a code logic bug — TypeError: Failed to fetch is always a network issue (blocked by CORS, DNS failure, HTTPS error, or connection refused). Check the Network panel, not just the Console, to see the actual HTTP status code and response. A 404 causes one error type, a CORS block causes another, and a network timeout causes a third — they can look similar in the Console. Conversely, a JavaScript runtime error (TypeError, ReferenceError) is always a code issue and appears with a stack trace pointing to the source file and line. The presence or absence of a stack trace is the fastest way to distinguish network-origin errors (no stack trace below the fetch call) from code-origin errors (full stack trace).