Union Types

A union type allows a value to belong to any one of several alternatives. Write it with a vertical bar, such as string | number.

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

A union is not a new runtime object. You can use only operations shared by every possible member until a check narrows the value. A function that accepts string | number must not blindly call a string-only method.

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

function label(value: string | number): string {
  if (typeof value === "number") return `ID-${value}`;
  return value.toUpperCase();
}
console.log(label(42));
console.log(label("guest"));

Verified output

ID-42
GUEST

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.

On 42, typeof detects number and the first branch returns ID-42. On "guest", the number condition fails and the string is converted to uppercase. Each branch has a narrower type.

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 call value.toUpperCase() before checking which member of string | number you received.

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.

Accept string | boolean and return a readable label for each member. Try both arguments and predict the output.

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.

Union Types: A union type allows a value to belong to any one of several alternatives.

On 42, typeof detects number and the first branch returns ID-42. On "guest", the number condition fails and the string is converted to uppercase. Each branch has a narrower type.

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