Why This Is Asked

Discriminated unions are one of TypeScript’s most powerful patterns. Interviewers want to see if you can model domain state safely using union types and ensure compile-time exhaustiveness — a key skill for building maintainable, type-safe applications.

Key Concepts

A discriminated union is a union of types sharing a common literal property (the “discriminant” or “tag”)
TypeScript narrows the type in switch/if blocks based on the discriminant value
Exhaustiveness checking via never: if a new variant is added, unhandled cases cause a compile error
Preferred over enums for modeling state machines (e.g., { status: "loading" } | { status: "error", message: string })
Works with instanceof narrowing for class hierarchies, but literal discriminants are more common in TS

Expected Answer

A Discriminated Union is a pattern where several types share a common property (the “discriminant”) with a literal value. TypeScript’s narrowing logic uses this property to differentiate between members of the union.

Example

interface Circle { kind: "circle"; radius: number; }
interface Square { kind: "square"; side: number; }
type Shape = Circle | Square;

Exhaustiveness Checking

To ensure a switch or if block handles every member of a union, you can use the never type. If a variable’s type is narrowed to never, it means every possible case has been handled.

function getArea(s: Shape) {
  switch (s.kind) {
    case "circle": return Math.PI * s.radius ** 2;
    case "square": return s.side * s.side;
    default:
      const _exhaustiveCheck: never = s;
      return _exhaustiveCheck;
  }
}

If you add a Triangle to the Shape union later, the default block will fail to compile because s will be of type Triangle, which is not assignable to never.

Common Mistakes

Implicit Narrowing: Relying on magic strings instead of a literal type discriminant.
Forgetting the never check: Losing compile-time safety when the union is expanded in the future.

Follow-up Questions

How does this differ from instanceof narrowing?
Can you use a boolean as a discriminant? (Answer: Yes, but it only allows for two states).

Discriminated Unions