Unhandled

Unhandler is a term used in computing to describe a condition, event, or error for which a program provides no code path to deal with it. In practice, it most often refers to an exception or error that is thrown but not caught or handled, allowing the runtime to terminate a program or leave it in an unpredictable state.

In languages that use exceptions, an unhandled exception occurs when code throws an exception without an enclosing try-catch block. In asynchronous environments such as JavaScript, an unhandled promise rejection happens when a promise is rejected and no rejection handler is attached. Some systems also consider unreturned error codes or missing error checks as unhandled, particularly in languages that return error values instead of throwing.

Consequences include abrupt termination, data loss, and security or reliability risks, and they can complicate debugging by obscuring the original cause. Unhandled conditions receive attention mainly after failure, rather than through proactive design.

Prevention relies on defensive programming: validating inputs, performing explicit error checks, and ensuring all new failure paths are handled. Common strategies include structured exception handling, central error hubs, logging, fail-fast design, and thorough testing such as unit tests, integration tests, and fuzzing. Static analysis and formal verification can also help identify unhandled paths before release.