Result Types for Error Handling

A Result union represents expected success or failure as ordinary data instead of throwing for every domain outcome. A discriminant such as ok lets TypeScript narrow the corresponding fields and forces the caller to consider both cases.

Reading the code: parsePort returns a tagged union. The first input converts to a positive integer and prints 8080. The second becomes NaN, so the error branch prints invalid port.

Keep the boundary clear: A Result is only as reliable as its validation logic. Converting input with Number alone does not ensure the value is within a useful port range.

Save the code as result-types-error-handling.ts. The result below was captured after compiling this exact example with strict TypeScript checking and executing the emitted JavaScript in Node.js.

TypeScript code

type Result<T> = { ok: true; value: T } | { ok: false; error: string };
function parsePort(text: string): Result<number> {
  const value = Number(text);
  return Number.isInteger(value) && value > 0 ? { ok: true, value } : { ok: false, error: "invalid port" };
}
for (const input of ["8080", "oops"]) {
  const result = parsePort(input);
  console.log(result.ok ? result.value : result.error);
}

Verified output

8080
invalid port

Build: npx tsc --strict --target es2022 --module commonjs --lib es2022,dom,esnext.disposable result-types-error-handling.ts; run: node result-types-error-handling.js. The exact filename may be changed freely. When a multi-file project exists, prefer its tsconfig.json and build scripts.

parsePort returns a tagged union. The first input converts to a positive integer and prints 8080. The second becomes NaN, so the error branch prints invalid port.

The output shown above comes from the initialized values in this program. Replacing a type annotation alone does not transform a JavaScript value.

A Result is only as reliable as its validation logic. Converting input with Number alone does not ensure the value is within a useful port range.

Check both the TypeScript diagnostic and the emitted JavaScript behavior. A program can compile correctly and still need runtime validation or error handling.

Restrict the valid range to 1–65535 and return a typed result for "70000".

Before running your change, write down the type you expect TypeScript to infer and the output you expect JavaScript to print. Then check both predictions.

A Result union represents expected success or failure as ordinary data instead of throwing for every domain outcome.

Try the practice change and re-run the exact program to test your understanding.