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In Express.js, middleware can handle error messages, usually through the following steps:

• Error Catch: In middleware, use the try...catch statement to catch errors in asynchronous operations, or use the if statement to check error conditions in synchronous operations.

• Error Object: After catching the error, create an error object, which can be a simple error string or an Error object with more information.

• Error Handling Middleware: Express.js allows you to define middleware that specifically handles errors. These middlewares accept four parameters: err, req, res, and next.

• Call next: When an error is encountered in a normal middleware, pass the error object to the next function, such as next(new Error('Something went wrong')), passing control to the next middleware.

• Error Response: In the error handling middleware, set the response status code and send an error message to the client, such as res.status(500).send('Internal Server Error').

• Log Logging: When handling errors, log error information to the log for developers to debug and fix issues.

• Error passing: If the error handling middleware does not handle the error, you can call next(err) without doing anything, so that Express.js’ default error handling middleware can handle it.

In summary, Express.js middleware handles error information by catching errors, creating error objects, using error handling middleware, setting responses, and logging. Error handling middleware is specially used to handle errors, receive errors through four parameters, and is responsible for sending appropriate responses to the client.

Error handling middleware is a specialized mechanism for handling errors in Express.js, which ensure proper response and logging of errors, thereby improving application robustness and maintainability.