Discriminated Unions

A discriminated union gives each alternative a shared property with a distinct literal value, so checking that property identifies the object shape.

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

The shared discriminant, such as kind, connects a variant to its relevant fields. This pattern is useful for UI states, API results, and events. Treat a state as a complete alternative instead of keeping unrelated optional properties everywhere.

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 =
  | { kind: "success"; value: number }
  | { kind: "failure"; message: string };
function report(result: Result): string {
  switch (result.kind) {
    case "success": return `Value: ${result.value}`;
    case "failure": return `Error: ${result.message}`;
  }
}
console.log(report({ kind: "success", value: 8 }));
console.log(report({ kind: "failure", message: "offline" }));

Verified output

Value: 8
Error: offline

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.

kind is success for the first object, making value safely accessible. For the second, kind is failure, so message is available. TypeScript connects each string discriminant to the correct fields.

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.value outside the success branch; a failure variant does not have that 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.

Model loading, loaded, and failed data states using a state discriminant, and print a different message for each.

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.

Discriminated Unions: A discriminated union gives each alternative a shared property with a distinct literal value, so checking that property identifies the object shape.

kind is success for the first object, making value safely accessible. For the second, kind is failure, so message is available. TypeScript connects each string discriminant to the correct fields.

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