Handling errors in Node.js is crucial for creating robust and reliable applications. Here’s a comprehensive guide on how to handle errors effectively in Node.js.

1. Types of Errors

Errors in Node.js can be broadly categorized into three types:

Synchronous Errors: These occur during the execution of synchronous code.
Asynchronous Errors: These occur in asynchronous operations, such as callbacks, promises, and async/await.
Operational Errors: These are anticipated errors such as network issues, invalid user input, etc.
Programmer Errors: Bugs in the code, such as undefined variables, type errors, etc.

2. Basic Error Handling

Try-Catch Block

Use try-catch blocks to handle synchronous errors.

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

Callback Error Handling

Pass errors to the callback function to handle errors in asynchronous code.

function someAsyncFunction(callback) {
  fs.readFile('/file-does-not-exist', (err, data) => {
    if (err) {
      return callback(err);
    }
    callback(null, data);
  });
}

someAsyncFunction((err, data) => {
  if (err) {
    console.error('Error occurred:', err.message);
  } else {
    console.log(data);
  }
});

3. Handling Promises

Using `.catch()`

Handle errors in promises with `.catch()`.

somePromiseFunction()
  .then(result => {
    // Handle result
  })
  .catch(error => {
    // Handle error
    console.error('Error occurred:', error.message);
  });

Using `try-catch` with `async/await`

Handle errors in async functions using `try-catch`.

async function someAsyncFunction() {
  try {
    let result = await somePromiseFunction();
    // Handle result
  } catch (error) {
    // Handle error
    console.error('Error occurred:', error.message);
  }
}

someAsyncFunction();

4. Global Error Handling

Uncaught Exceptions

Handle uncaught exceptions globally.

process.on('uncaughtException', (error) => {
  console.error('Uncaught Exception:', error.message);
  // Consider shutting down the process safely
});

Unhandled Promise Rejections

Handle unhandled promise rejections globally.

process.on('unhandledRejection', (reason, promise) => {
  console.error('Unhandled Rejection at:', promise, 'reason:', reason);
  // Consider shutting down the process safely
});

5. Creating Custom Error Classes

Create custom error classes for more specific error handling.

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

try {
  throw new CustomError('This is a custom error');
} catch (error) {
  console.error(error.name); // CustomError
  console.error(error.message); // This is a custom error
}

6. Error Handling Middleware in Express.js

In an Express.js application, use error-handling middleware to handle errors.

const express = require('express');
const app = express();

// Middleware to handle errors
app.use((err, req, res, next) => {
  console.error(err.stack);
  res.status(500).send('Something broke!');
});

// Example route that triggers an error
app.get('/', (req, res) => {
  throw new Error('BROKEN'); // Express will catch this on its own
});

app.listen(3000, () => {
  console.log('Server is running on port 3000');
});

7. Best Practices

Graceful Shutdown: Ensure the application can shut down gracefully when an error occurs.
Logging: Use proper logging mechanisms (e.g., Winston, Bunyan) to log errors.
Validation: Validate inputs to prevent errors.
Monitoring: Use monitoring tools (e.g., New Relic, Sentry) to keep track of errors in production.

By following these practices and utilizing the different error-handling mechanisms provided by Node.js, you can create resilient applications that can handle errors gracefully and maintain stability.

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How to Handle Errors in Node.js

#node.js

#handle errors