TypeScript Type Narrowing & Discriminated Unions: Interview Questions & Code

TypeScript's powerful type inference engine relies on Type Narrowing—the process of refining a broad type into a more specific type within a conditional code block.

1. Type Guards: typeof, instanceof, and in

function formatInput(val: string | number | Date) {
  if (typeof val === "string") {
    return val.toUpperCase(); // Narrowed to string
  }
  if (val instanceof Date) {
    return val.toISOString(); // Narrowed to Date
  }
  return val.toFixed(2); // Narrowed to number
}

2. Custom Type Guard Functions (is Keyword)

interface User { id: string; name: string; }

function isAdmin(user: User): user is Admin { return "permissions" in user && Array.isArray((user as Admin).permissions); } ```

3. Discriminated Unions & Exhaustiveness Checking

A Discriminated Union is a union type where every member shares a common literal property (the discriminant).

type NetworkState =
  | { status: "idle" }
  | { status: "loading" }
  | { status: "success"; data: string[] }

function renderUI(state: NetworkState): string { switch (state.status) { case "idle": return "Ready"; case "loading": return "Loading..."; case "success": return Loaded ${state.data.length} items; case "error": return Error: ${state.error.message}; default: { const _exhaustiveCheck: never = state; return _exhaustiveCheck; } } } ```

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