result-guard

Wraps a throwing function so it returns a neverthrow Result/ResultAsync instead. See the project README for the full problem statement and design reasoning.

Install

npm install result-guard neverthrow

Usage

import { guard } from "result-guard";

async function fetchUser(id: string) {
  if (!id) throw new Error("missing id");
  return { id, name: "Alice" };
}

const guardedFetch = guard(fetchUser);

const result = await guardedFetch("123");
result.match(
  (user) => console.log(user),
  (error) => console.error(error.kind === "error" ? error.message : "unknown failure")
);

By default, guard() gives you Result<T, DefaultGuardError> — { kind: "error", name, message, cause } if the caught value was a real Error, or { kind: "unknown", cause } otherwise. Pass your own error mapper to fully replace it with typed errors specific to your domain:

type ApiError = { kind: "not-found" } | { kind: "server-error"; status: number };

const guardedFetch = guard(fetchUser, (e): ApiError =>
  e instanceof NotFoundError ? { kind: "not-found" } : { kind: "server-error", status: 500 }
);

Decorator

For class-based code:

class UserService {
  @ResultGuard()
  async fetchUser(id: string) {
    if (!id) throw new Error("missing id");
    return { id, name: "Alice" };
  }
}

Sync methods stay synchronous — guard() never forces a Promise onto a function that wasn't already async.

License

MIT