How do I create custom error handling middleware in Express.js?

How do I create custom error handling middleware in Express.js?

To create custom error handling middleware in Express.js, you need to define a middleware function that takes four arguments: err, req, res, and next. Here's a basic example:

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

This middleware will log the error stack and send a 500 response to the client.

Can I customize the error response based on the error type?

Yes, you can customize the error response by checking the type of error. For example:

app.use((err, req, res, next) => {
    if (err instanceof ValidationError) {
        return res.status(400).json({ message: err.message });
    }
    res.status(500).send('Internal Server Error');
});

What if I want to log errors to a file instead of the console?

You can use a logging library like winston to log errors to a file. Here's how you can set it up:

const winston = require('winston');

const logger = winston.createLogger({
    transports: [
        new winston.transports.File({ filename: 'error.log' })
    ]
});

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