Type Narrowing Examples

Type narrowing turns a union into a more specific type after a check TypeScript can understand. A discriminated union gives each variant a distinct tag, making an exhaustive switch predictable and safe.

The kind field selects a variant, so user or amount is accessible in its corresponding branch. The never assignment flags missing cases if the union is extended.

Checking merely that an object exists does not establish which union variant it is; check the discriminant before property access.

This source was compiled using TypeScript 5.8.3 with strict checking, then executed with Node.js 22.16.0. Its stdout below is the actual captured output. The core logic is runnable without a framework or a remote API; integration-specific examples appear in a separate lab when needed.

TypeScript source

type Event = { kind: "login"; user: string } | { kind: "purchase"; amount: number };
function describe(event: Event): string {
  switch (event.kind) {
    case "login": return `Login: ${event.user}`;
    case "purchase": return `Purchase: ${event.amount}`;
    default: { const neverEvent: never = event; return neverEvent; }
  }
}
console.log(describe({ kind: "login", user: "Maya" }));
console.log(describe({ kind: "purchase", amount: 40 }));

Verified output

Login: Maya
Purchase: 40

Compile: npx tsc --strict --target es2022 --module commonjs type-narrowing-examples.ts; run: node type-narrowing-examples.js. For examples importing node: modules, install @types/node as a development dependency; for browser or React projects use the separate lab setup.

The kind field selects a variant, so user or amount is accessible in its corresponding branch. The never assignment flags missing cases if the union is extended.

Before editing the code, predict what the output will be for one changed input. Run it and compare your prediction with the actual result.

Checking merely that an object exists does not establish which union variant it is; check the discriminant before property access.

Determine whether the issue belongs to the type system, a trust boundary, or actual runtime behavior before choosing a correction.

Add a logout variant and observe the compile error until the switch handles it.

Record expected success and failure outcomes, run the code, and compare the observed behavior. Keep the exercise compatible with strict type checking.

The kind field selects a variant, so user or amount is accessible in its corresponding branch. The never assignment flags missing cases if the union is extended.

Apply the same reasoning to the practice task, then integrate it into a real app only when the runtime and dependency setup are ready.