Handling Errors

Error types are inferred

When you create a workflow, TypeScript computes the error union from your dependencies:

const fetchUser = async (id: string): AsyncResult<User, 'NOT_FOUND'> => { ... };
const fetchPosts = async (id: string): AsyncResult<Post[], 'FETCH_ERROR'> => { ... };
const sendEmail = async (to: string): AsyncResult<void, 'EMAIL_FAILED'> => { ... };

const workflow = createWorkflow('workflow', { fetchUser, fetchPosts, sendEmail });

const result = await workflow.run(async ({ step, deps }) => { ... });
// result.error is: 'NOT_FOUND' | 'FETCH_ERROR' | 'EMAIL_FAILED' | UnexpectedError

Add a new dependency? The error union updates automatically.

UnexpectedError

If code throws an exception (not a returned error), it becomes an UnexpectedError (a TaggedError). The original thrown value is in error.cause:

import { isUnexpectedError } from 'awaitly';

const badOperation = async (): AsyncResult<string, 'KNOWN_ERROR'> => {
  throw new Error('Something broke'); // Throws instead of returning err()
};

const workflow = createWorkflow('workflow', { badOperation });
const result = await workflow.run(async ({ step, deps }) => {
  return await step('badOperation', () => badOperation());
});

if (!result.ok && isUnexpectedError(result.error)) {
  console.log(result.error.cause); // The original Error object
}

Wrapping throwing code

Use step.try to convert thrown exceptions into typed errors:

const result = await workflow.run(async ({ step, deps }) => {
  const data = await step.try(
    'fetchData',
    async () => {
      const res = await fetch('/api/data');
      if (!res.ok) throw new Error(`HTTP ${res.status}`);
      return res.json();
    },
    { error: 'FETCH_FAILED' as const }
  );

  return data;
});
// result.error includes 'FETCH_FAILED'

Preserving error details

If your function returns rich error objects, use step.fromResult:

type ApiError = { code: string; message: string };

const callApi = async (): AsyncResult<Data, ApiError> => {
  return err({ code: 'RATE_LIMITED', message: 'Too many requests' });
};

const result = await workflow.run(async ({ step, deps }) => {
  const data = await step.fromResult(
    'callApi',
    () => callApi(),
    {
      onError: (apiError) => ({
        type: 'API_ERROR' as const,
        code: apiError.code,
        message: apiError.message,
      }),
    }
  );
  return data;
});

Handling specific errors

Use a switch statement for exhaustive handling:

if (!result.ok) {
  switch (result.error) {
    case 'NOT_FOUND':
      return res.status(404).json({ error: 'User not found' });
    case 'UNAUTHORIZED':
      return res.status(401).json({ error: 'Please log in' });
    case 'FETCH_ERROR':
      return res.status(502).json({ error: 'Upstream service failed' });
    default:
      // UnexpectedError or unknown error
      console.error(result.error);
      return res.status(500).json({ error: 'Internal error' });
  }
}

TypeScript ensures you handle all known error cases.

Custom unexpected errors

By default, thrown exceptions become an UnexpectedError. With run(), use ErrorsOf to derive error types from your deps:

import { type ErrorsOf } from 'awaitly';

const deps = { fetchUser, fetchPosts };
type RunErrors = ErrorsOf<typeof deps>;

const result = await run<User, RunErrors>(async ({ step }) => {
  const user = await step('fetchUser', () => fetchUser('1'));
  const posts = await step('fetchPosts', () => fetchPosts(user.id));
  return user;
});
// result.error is: 'NOT_FOUND' | 'FETCH_ERROR' | UnexpectedError

To replace UnexpectedError with a custom type, pass catchUnexpected:

const workflow = createWorkflow('workflow', { fetchUser, fetchPosts },
  {
    catchUnexpected: (thrown) => ({
      type: 'UNEXPECTED' as const,
      message: String(thrown),
    }),
  }
);
// result.error is now your workflow errors | { type: 'UNEXPECTED', message: string }

When to use string literals vs objects

Use case	Recommendation
Simple distinct states	String literals: 'NOT_FOUND' | 'UNAUTHORIZED'
Errors with context	Objects: { type: 'NOT_FOUND', id: string }
API responses	Tagged Errors for structured data

Need help?

Having issues with TypeScript narrowing or error handling? See Troubleshooting.

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