Express.js Error Handling

Last updated on July 11, 2023

In Express.js, error handling can be done using middleware functions. When an error occurs in your application, Express.js will pass the error object to the next error-handling middleware function. Here’s an example of how you can implement error handling in Express.js.

error-handling middleware

Let’s create an error-handling middleware function.

app.use((err, req, res, next) => {
  // Default status code to 500 if it's not set
  const statusCode = res.statusCode !== 200 ? res.statusCode : 500;

  // Set the status code and send the error message
  res.status(statusCode).json({
    message: err.message,
    stack: process.env.NODE_ENV === 'production' ? '' : err.stack
  });
});

How to use the error-handling middleware function

Use the error-handling middleware function in your application

// Define your routes and middleware functions
app.get('/', (req, res, next) => {
  // Example error, assuming `someFunction` throws an error
  try {
    someFunction();
  } catch (err) {
    next(err); // Pass the error to the error-handling middleware
  }
});

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

In this example, when an error occurs in the route handler (/), the next(err) statement is called to pass the error object to the error-handling middleware. The middleware function then sets the appropriate status code and sends a JSON response with the error message.

It’s important to define the error-handling middleware after all other routes and middleware functions in your application, as Express.js will sequentially pass the error to the next error-handling middleware function if one exists.

Remember to replace someFunction() with your actual code that might throw an error. You can customize the error-handling middleware to suit your needs, such as logging the error or providing additional error details based on your application’s requirements.