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Error Handling in Node.js

Error Handling in Node.js

Error handling is a crucial aspect of writing robust and reliable Node.js applications. It involves capturing and managing errors that may occur during the execution of your code. Here are some key concepts and practices for error handling in Node.js:

1. Synchronous Error Handling:

In synchronous code, errors can be caught using try-catch blocks.

try {
  // Code that may throw an error
  const result = someSynchronousFunction();
} catch (error) {
  // Handle the error
  console.error('An error occurred:', error.message);
}
 

2. Asynchronous Error Handling:

For asynchronous code, you should use callback functions or Promises to handle errors.

Callbacks:

const fs = require('fs');

fs.readFile('nonexistent-file.txt', 'utf8', (err, data) => {
  if (err) {
    console.error('Error reading file:', err.message);
    return;
  }
  console.log(data);
});
 

Promises:

const fs = require('fs').promises;

fs.readFile('nonexistent-file.txt', 'utf8')
  .then(data => console.log(data))
  .catch(error => console.error('Error reading file:', error.message));
 

3. Custom Error Classes:

You can create custom error classes to represent specific types of errors in your application. This can make it easier to identify and handle different types of errors.

class CustomError extends Error {
  constructor(message, status) {
    super(message);
    this.name = 'CustomError';
    this.status = status || 500;
  }
}

try {
  throw new CustomError('Something went wrong', 400);
} catch (error) {
  console.error(error.name, error.message);
  console.error('Status:', error.status);
}
 

4. Error Middleware in Express:

In an Express application, you can use error-handling middleware to catch errors that occur during the request-response cycle.

app.use((err, req, res, next) => {
  console.error(err.stack);
  res.status(500).send('Something went wrong!');
});
 

5. Handling Promise Rejections:

To handle unhandled promise rejections, you can use the unhandledRejection event.

process.on('unhandledRejection', (reason, promise) => {
  console.error('Unhandled Rejection at:', promise, 'reason:', reason);
  // Handle the error here
});
 

6. Logging Errors:

It's important to log errors to help with debugging and troubleshooting.

try {
  // Code that may throw an error
} catch (error) {
  console.error('An error occurred:', error.message);
  // Log the error to a file or a monitoring service
}
 

7. Graceful Shutdown:

Handle process signals to perform cleanup operations before the application exits due to an error.

process.on('SIGINT', () => {
  // Clean up resources and exit
  process.exit();
});
 

8. Testing Error Cases:

When writing unit tests, ensure that you cover error cases to validate that your code handles them correctly.