TypeScript Promise Helper

Go-style [err, result] tuple instead of try/catch per call. Non-Error rejections (strings, plain objects) are wrapped so err is always an Error.

export const to = async <T>(promise: Promise<T>): Promise<[null, T] | [Error, null]> => {
  try {
    return [null, await promise];
  } catch (err) {
    return [err instanceof Error ? err : new Error(String(err)), null];
  }
};

const [err, user] = await to(fetchUser(id));
if (err) return res.status(502).json({ message: err.message });
// user is narrowed to T here

Check:

import assert from 'node:assert/strict';
 
assert.deepEqual(await to(Promise.resolve(42)), [null, 42]);
const [err] = await to(Promise.reject(new Error('boom')));
assert.equal(err?.message, 'boom');
const [err2] = await to(Promise.reject('string-reason'));
assert.equal(err2?.message, 'string-reason');