Exhaustive Checking with never

Exhaustive type checking uses the never type to ensure every possible member of a discriminated union is handled inside switch or if statements.

1. Introduction & Architecture

2. Deep Dive & Core Concepts

If all union cases are handled, the variable in the default case is narrowed to type never. If a new union member is added later without updating the switch statement, TypeScript reports a compile error at the assertNever call site.

3. Basic Code Example

4. Advanced Production Patterns

5. Interactive Code Playground

type Status = "idle" | "loading" | "success";
function render(status: Status) {
  switch (status) {
    case "idle": return "Idle";
    case "loading": return "Loading";
    case "success": return "Success";
    default:
      const _exhaustive: never = status;
      return _exhaustive;
  }
}
console.log(render("idle"));

6. Common Pitfalls & Edge Cases

7. Interview Q&A & Quizzes

Q: How does exhaustive type checking work in TypeScript?

A: By assigning the unhandled conditional variable to a never typed variable in the fallback branch, triggering a build error if any union member remains unhandled.

8. Summary Comparison Table