TypeScript · Middle · Question 2 of 75

How do you guarantee that a `switch` handled every member of a union?

function area(s: Shape) {
  switch (s.kind) {
    case 'circle': return Math.PI * s.radius ** 2;
    case 'square': return s.size ** 2;
    default: { const _exhaustive: never = s; return _exhaustive; }
  }
}

✓Correct answer: Assign the value in default to a variable of type never
2Add a // @ts-exhaustive comment before the switch
3Enable the strictSwitch flag in compilerOptions

Explanation

If you add a new shape to Shape and forget a case, a non-empty type remains in default, and assigning it to never causes a compile error. The noImplicitReturns flag and the ESLint rule switch-exhaustiveness-check also help.

Further reading

TS Handbook: Narrowing ↗

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