Error handling in Express Application

Error Handling refers to how Express catches and processes errors that occur both synchronously and asynchronously. Express comes with a default error handler so you don’t need to write your own to get started.

An error can be occurred in an Express application in the following two cases.

1.      When requested route is not found in server

2.      When the request handling middleware generates an error.

Handling unavailable routes 

 Express Application creates a routing table in the order of route handlers in the application. When a client makes a request,It can start comparing the request handlers in the order of they derived.

If no match found,then it returns an HTML response with error message in it. We can handle this error by adding a middleware as last one in the application.

               app.use( (req, res, next) = > {

               res.send(`path ${ req.url } of type request ${ req.method }is not found`);

});

This middleware function can execute for every client’s request. So , by placing this as last one,we can make it respond only for unavailable paths.

Handling errors of request handling middlewares

Whenever an error is occurred ,the default Error handling middleware of express can deal with that.

The default error handling middleware take four objects like below.

                               (error, request , response , next )=>{}

When an error is occurred in execution of request, the Express creates an Error objects and handover it to default error handling mechanism by suspending the execution rest of code in that request handler.

One can throw an Error object to that default error handler by using

                                                 let err=new Error(“error message”);

                                                                 next(err); //this will call error handling middleware

Note: next() will call next middleware in the list where as next(err) will call error handling middleware

Now add the following error handling middleware as last one in the list and just above list() method.

                                               app.use( (err , req , res , next )= > { } );

So the final order of route handlers in express app should like below

                               //write all request handlers

                               //write middleware to deal with unavailable paths

                                               app.use( (req, res, next) = > {

               res.send(`path ${ req.url } of type request ${ req.method }is not found`);

});

                               //write error handling middleware

                               app.use( (err , req , res , next )= > {

               console.log(err.stack);

               res.send(“something went wrong!!!!”);

 } );

                               //assign port number

                               app.listen(some port number here);