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never type

Represents values that never occur, such as unreachable code paths.

Since TS 2.0 Spec ↗

Syntax

never

Examples

function fail(msg: string): never {
  throw new Error(msg);
}

function exhaustive(x: never): never {
  throw new Error('unhandled: ' + x);
}
type Shape = { kind: 'a' } | { kind: 'b' };
function area(s: Shape) {
  switch (s.kind) {
    case 'a': return 1;
    case 'b': return 2;
    default: return exhaustive(s);
  }
}

Notes

`never` is the type of functions that never return (throw or infinite loop) and of impossible values. It is the bottom type, assignable to every type but with no assignable values. Empty unions reduce to `never`; use it for exhaustiveness checks in switches.

See also