How to fix UnhandledPromiseRejectionWarning: undefined (no error reason provided) in Node.js

// Bad - rejecting with undefined
Promise.reject();
Promise.reject(undefined);
reject(); // In promise executor

// Good - reject with Error object
Promise.reject(new Error('Operation failed'));
reject(new Error('Invalid input provided'));

// Also good - reject with meaningful value
Promise.reject('Database connection timeout');
reject({ code: 'INVALID_INPUT', field: 'email' });

// Bad - implicit undefined return
async function fetchUser(id) {
  if (!id) {
    throw; // Throws undefined
  }
  return await db.users.findById(id);
}

// Good - explicit error
async function fetchUser(id) {
  if (!id) {
    throw new Error('User ID is required');
  }
  return await db.users.findById(id);
}

// Bad - losing error information
fetch('/api/data')
  .catch(err => {
    console.error('Fetch failed');
    return Promise.reject(); // Rejects with undefined
  });

// Good - preserving error
fetch('/api/data')
  .catch(err => {
    console.error('Fetch failed:', err);
    return Promise.reject(err); // Pass error forward
  });

// Better - throw to propagate
fetch('/api/data')
  .catch(err => {
    console.error('Fetch failed:', err);
    throw err; // Automatically rejects with err
  });

process.on('unhandledRejection', (reason, promise) => {
  if (reason === undefined) {
    console.error('⚠️  Promise rejected with undefined!');
    console.error('Promise:', promise);
    console.error('Stack trace to find source:');
    console.error(new Error().stack);
  } else {
    console.error('Unhandled rejection:', reason);
  }
});

// Bad - reject called without argument
new Promise((resolve, reject) => {
  if (someCondition) {
    reject(); // Rejects with undefined
  }
  resolve(result);
});

// Good - reject with error
new Promise((resolve, reject) => {
  if (someCondition) {
    reject(new Error('Condition not met'));
  }
  resolve(result);
});

// Better - use try/catch
new Promise((resolve, reject) => {
  try {
    if (!someCondition) {
      throw new Error('Condition not met');
    }
    resolve(result);
  } catch (err) {
    reject(err);
  }
});

// .eslintrc.json
{
  "rules": {
    "prefer-promise-reject-errors": ["error", {
      "allowEmptyReject": false
    }],
    "@typescript-eslint/no-floating-promises": "error",
    "no-throw-literal": "error"
  }
}