#NonExhaustiveError ts-pattern

The runtime error thrown by .exhaustive() when a value reaches it that no pattern matched.

match(value).with(...).exhaustive() // throws NonExhaustiveError if no arm matched at runtime

#When this matters

.exhaustive() is checked at compile time by TypeScript — if your patterns cover the input type, the call type-checks and the runtime branch is unreachable in well-typed code. NonExhaustiveError is the safety net for two cases:

1. Type assertion holes — a value typed as T that's actually something else at runtime (deserialization, external APIs).
2. Use with any — if you opt out of exhaustiveness checking by passing through any, the runtime check still fires.

Treat a NonExhaustiveError in production as a type-safety bug: somewhere upstream, the value is wider than its declared type.

import { match, NonExhaustiveError } from 'massaman'

try {
  return match(event).with(...).exhaustive()
} catch (e) {
  if (e instanceof NonExhaustiveError) {
    logger.error({ value: e.input }, 'pattern match escaped at runtime')
  }
  throw e
}

#See also

• match
• ts-pattern README — Exhaustiveness