TypeScript 7 reference · Type Operators and Keywords

TypeScript never Keyword

The type with no values: for functions that never return, impossible branches, and exhaustiveness checks.

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Examples

Exhaustiveness checkingTry it yourself

type Shape = { kind: "circle"; r: number } | { kind: "square"; s: number };
function area(shape: Shape): number {
  switch (shape.kind) {
    case "circle": return Math.PI * shape.r ** 2;
    case "square": return shape.s ** 2;
    default: {
      const unreachable: never = shape; // errors if a new kind is added and not handled
      return unreachable;
    }
  }
}
console.log(area({ kind: "square", s: 3 }));

Output

Functions that never returnTry it yourself

function fail(message: string): never {
  throw new Error(message);
}
try {
  fail("not implemented");
} catch (e) {
  console.log((e as Error).message);
}

Output

not implemented

Version history

Version	Change
TypeScript 2.0	Added

unknown · The type-safe 'anything': a value of type unknown must be narrowed (checked) before you can use it, unlike any.
Exclude<UnionType, ExcludedMembers> · Remove members from a union type. Exclude<'a' | 'b' | 'c', 'a'> is 'b' | 'c'.
TypeScript keyof, typeof and Indexed Access · tutorial

Examples

Version history

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