Function unreachable

unreachable(x: never): never

Throws a TypeScript error if an unreachable case is reached. Often used to ensure that a switch statement is exhaustive.

Parameters

x: never
The value that should never be reached

Returns never

Throws an error

Example

type PossibleValues = 'a' | 'b';

const handleValue = (value: PossibleValues) => {
  switch (value) {
    case 'a':
      return 'A';
    case 'b':
      return 'B';
    default:
      return unreachable(value);
  }
}

If 'c' is added to PossibleValues but not handled in the switch statement,
TypeScript will throw an error.