A Result type in TypeScript

Errors in the return type, so the caller cannot forget them.

type Result<T, E = Error> =
  | { ok: true; value: T }
  | { ok: false; error: E };

const ok = <T>(value: T): Result<T, never> => ({ ok: true, value });
const err = <E>(error: E): Result<never, E> => ({ ok: false, error });

function parsePort(raw: string): Result<number, string> {
  const port = Number.parseInt(raw, 10);
  if (Number.isNaN(port)) return err(`not a number: ${raw}`);
  if (port < 1 || port > 65535) return err(`out of range: ${port}`);
  return ok(port);
}

function map<T, U, E>(result: Result<T, E>, fn: (value: T) => U): Result<U, E> {
  return result.ok ? ok(fn(result.value)) : result;
}

const parsed = parsePort("8080");
console.log(parsed.ok ? parsed.value : parsed.error);
console.log(map(parsePort("99999"), (n) => n * 2));

How it works

1. A discriminated union makes the two outcomes explicit.
2. Narrowing on ok gives you the value or the error.
3. Helpers keep the construction short.

Keywords and builtins used here

• E
• Number
• Result
• T
• const
• false
• function
• if
• isNaN
• number
• parseInt
• return
• string
• true
• type

The run, in numbers

Lines

21

Characters to type

739

Tokens

261

Three-star pace

105 tpm

At the three-star pace of 105 tokens a minute, this run takes about 149 seconds.

Type this snippet

Step 1 of 4 in Results & validation, step 5 of 19 in Classes & patterns.

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