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Exhaustive check through never

TypeScriptJunior~1 min read
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Short answer: make a discriminated union and add a never-based "guard" to the switch. Then, when a new variant is added, TypeScript will raise a compile error.

The basic technique (exhaustive check through never)

javascript
type Shape = | { kind: "circle"; radius: number } | { kind: "square"; side: number } | { kind: "triangle"; base: number; height: number }; function area(s: Shape): number { switch (s.kind) { case "circle": return Math.PI * s.radius ** 2; case "square": return s.side ** 2; case "triangle": return (s.base * s.height) / 2; default: // if a new variant appears, s will NOT be 'never' here → error const _exhaustive: never = s; throw new Error(`Unhandled case: ${_exhaustive}`); } }
  • When you add, for example, { kind: "rectangle" }, s in default stops being never → a compile error forces you to add the case.

A variant with a helper function assertNever

Convenient if you do this check often:

javascript
function assertNever(x: never): never { throw new Error(`Unhandled case: ${String(x)}`); } function area(s: Shape): number { switch (s.kind) { case "circle": return Math.PI * s.radius ** 2; case "square": return s.side ** 2; case "triangle": return (s.base * s.height) / 2; default: return assertNever(s); // a type error fires here on a new variant } }

Without default: a "post-switch" guard

If you don't want a default, you can require every branch to return a value, and then check never after the switch:

javascript
function area(s: Shape): number { switch (s.kind) { case "circle": return Math.PI * s.radius ** 2; case "square": return s.side ** 2; case "triangle": return (s.base * s.height) / 2; } const _exhaustive: never = s; // fires when a new kind is added return _exhaustive; }

Useful settings and linting

  • The ESLint rule @typescript-eslint/switch-exhaustiveness-check highlights unclosed switch statements.
  • The compiler flag noFallthroughCasesInSwitch helps avoid fallthrough between cases (not about exhaustiveness, but useful).

Summary

To guarantee that a switch handles all variants of a union:

  1. Use a discriminant field (for example, kind).
  2. Add an exhaustive check by assigning to never (in default or after the switch).
  3. (Optionally) Extract the check into assertNever.

Short Answer

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