no-unhandled-promise

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Keywords: unhandled promise, promise rejection, async error handling, Promise, async/await, error handling, Node.js, JavaScript promises, async patterns, promise chains, try/catch, await, error propagation, CWE-1024, SonarQube RSPEC-4635

Disallow unhandled Promise rejections with LLM-optimized suggestions for proper async error handling. This rule detects promises that are created but never have

Disallow unhandled Promise rejections with LLM-optimized suggestions for proper async error handling. This rule detects promises that are created but never have their rejection handled, preventing silent failures in production applications.

Quick Summary

Aspect	Details
Severity	Error (production safety)
Auto-Fix	❌ No (complex logic)
Category	Error Handling
ESLint MCP	✅ Optimized for ESLint MCP integration
Best For	Production applications, async codebases
Suggestions	✅ Add .catch(), use try/catch, use await

Rule Details

Why This Matters

Issue	Impact	Solution
🔒 Silent Failures	Errors swallowed without logging	Proper error handling
🐛 Debugging	Hard to trace async failures	Centralized error handling
⚡ Reliability	Application crashes unexpectedly	Graceful error recovery
📊 Monitoring	No visibility into promise failures	Error tracking & alerting

Configuration

Option	Type	Default	Description
`ignoreInTests`	`boolean`	`true`	Skip promise checks in test files
`ignoreVoidExpressions`	`boolean`	`false`	Skip promises in void expressions

Examples

❌ Incorrect

// Unhandled promise rejection
fetchUserData(userId);

// Promise chain without error handling
apiCall()
  .then(processData)
  .then(saveToDatabase);

// Async function without await or error handling
async function processOrder(orderId) {
  validateOrder(orderId); // Promise not handled
  return orderId;
}

✅ Correct

// Handle with .catch()
fetchUserData(userId)
  .catch(error => {
    logger.error('Failed to fetch user data', { userId, error });
  });

// Complete promise chain
apiCall()
  .then(processData)
  .then(saveToDatabase)
  .catch(error => {
    logger.error('API pipeline failed', { error });
  });

// Use try/catch in async functions
async function processOrder(orderId) {
  try {
    await validateOrder(orderId);
    return orderId;
  } catch (error) {
    logger.error('Order validation failed', { orderId, error });
    throw error;
  }
}

// Or use await at the call site
async function handleOrder(orderId) {
  await processOrder(orderId); // Error will propagate
}

Configuration Examples

Basic Usage

// eslint.config.mjs
export default [
  {
    rules: {
      'reliability/no-unhandled-promise': 'error',
    },
  },
];

Ignore Test Files

{
  rules: {
    'reliability/no-unhandled-promise': ['error', {
      ignoreInTests: true  // Default: true
    }]
  }
}

Strict Mode (No Exceptions)

{
  rules: {
    'reliability/no-unhandled-promise': ['error', {
      ignoreInTests: false,
      ignoreVoidExpressions: false
    }]
  }
}

LLM-Optimized Output

🚨 [CWE-1024](https://cwe.mitre.org/data/definitions/1024.html) | Unhandled promise rejection detected | HIGH
   Fix: Add .catch() handler or use try/catch | https://rules.sonarsource.com/javascript/RSPEC-4635/

Suggestions:
  🔧 Add .catch(error => { ... })
  🔄 Use try/catch block
  ⚡ Use await in async function

Advanced Usage

Integration with Error Boundaries

// React component with error boundary
class ErrorBoundary extends React.Component {
  componentDidCatch(error: Error, errorInfo: React.ErrorInfo) {
    // Handle async errors from promises
    logger.error('React error boundary caught error', {
      error: error.message,
      componentStack: errorInfo.componentStack
    });
  }
}

// Usage
<ErrorBoundary>
  <AsyncComponent />
</ErrorBoundary>

Global Unhandled Rejection Handler

// Global handler for truly unhandled rejections
process.on('unhandledRejection', (reason, promise) => {
  logger.error('Unhandled promise rejection', {
    reason,
    promise: promise.toString()
  });
  // Don't exit process in production
});

When Not To Use

Scenario	Recommendation
🧪 Prototyping	Disable or use warning level
📚 Tutorials	Add to ignorePatterns
🔧 Build Scripts	Use ignorePatterns: ['scripts']

Comparison with Alternatives

Feature	no-unhandled-promise	eslint-plugin-promise	ESLint built-in
Promise Detection	✅ Static analysis	✅ Runtime checks	❌ No
Auto-Fix	❌ No	❌ No	❌ No
LLM-Optimized	✅ Yes	❌ No	❌ No
Customizable	✅ Yes	⚠️ Limited	❌ No

Error Message Format

🚨 [CWE-1024](https://cwe.mitre.org/data/definitions/1024.html) | Unhandled promise rejection detected | HIGH
   Fix: Add .catch() handler or use try/catch | https://rules.sonarsource.com/javascript/RSPEC-4635/

no-silent-errors - Detects empty catch blocks
no-missing-error-context - Requires error context information

Known False Negatives

The following patterns are not detected due to static analysis limitations:

Dynamic Variable References

Why: Static analysis cannot trace values stored in variables or passed through function parameters.

// ❌ NOT DETECTED - Value from variable
const value = externalSource();
processValue(value); // Variable origin not tracked

Mitigation: Implement runtime validation and review code manually. Consider using TypeScript branded types for validated inputs.

Wrapped or Aliased Functions

Why: Custom wrapper functions or aliased methods are not recognized by the rule.

// ❌ NOT DETECTED - Custom wrapper
function myWrapper(data) {
  return internalApi(data); // Wrapper not analyzed
}
myWrapper(unsafeInput);

Mitigation: Apply this rule's principles to wrapper function implementations. Avoid aliasing security-sensitive functions.

Imported Values

Why: When values come from imports, the rule cannot analyze their origin or construction.

// ❌ NOT DETECTED - Value from import
import { getValue } from './helpers';
processValue(getValue()); // Cross-file not tracked

Mitigation: Ensure imported values follow the same constraints. Use TypeScript for type safety.