Generic Type Aliases

A generic type alias can express reusable collections, unions, and conditional relationships involving a type parameter.

What you will learn: the purpose of generic type aliases, the rules TypeScript checks, and what its JavaScript behavior looks like in a runnable example.

Type aliases can name shapes or combinations that are awkward to express as an interface. A common example is Result<T>, where success carries a typed value and failure carries an error message.

Key distinction: TypeScript checks the declared types during development; emitted JavaScript carries out the value-level operation at runtime. A type declaration alone never validates untrusted runtime input.

Read this illustration from the first labeled part to the last. Compare it with the example and identify which parts are type checks and which parts run in JavaScript.

This self-contained example uses fixed values, so it runs without reading input or depending on an external service. Paste it into the TypeScript Playground or save it as main.ts.

TypeScript program

type Result<T> =
  | { ok: true; data: T }
  | { ok: false; error: string };
function display(result: Result<number>): string {
  return result.ok ? `Count: ${result.data}` : `Error: ${result.error}`;
}
console.log(display({ ok: true, data: 9 }));
console.log(display({ ok: false, error: "missing" }));

Verified output

Count: 9
Error: missing

Run locally: npx tsc --strict --target es2022 --module commonjs main.ts, then node main.js. For projects with a tsconfig.json, build using that configuration instead.

The boolean literal ok acts as the discriminant. When true, data is a number and prints Count: 9. When false, error is a string and prints Error: missing.

Check each step: identify the input values, follow the JavaScript operation in execution order, and match the final expression with the exact output shown. An annotation can reject invalid source before execution, but does not change the value printed.

Do not access result.data before narrowing result.ok; a failure variant does not promise a data property.

How to debug: reproduce the issue in a minimal example. For compiler errors, inspect the inferred and declared types; for a runtime surprise, log the actual values and follow their references or branches.

Change Result<number> to Result<string> in a formatter and test both alternatives.

Practice routine: predict the result on paper, edit one part of the code, compile under --strict, run the JavaScript, and explain whether TypeScript caught an error or JavaScript simply behaved differently.

Generic Type Aliases: A generic type alias can express reusable collections, unions, and conditional relationships involving a type parameter.

The boolean literal ok acts as the discriminant. When true, data is a number and prints Count: 9. When false, error is a string and prints Error: missing.

Before continuing, explain one valid use case, one common pitfall, and the separation between compile-time checking and runtime behavior for this feature.