Handling error in node.js & express

Question

I build a server with node.js and Express.

Everything works great, but in some cases the client sends invalid parameter, that I don't predict, and I don't handle with. The result of these requests that my server failed, and can't serve other requests.

For example, In one of my function. I have the next lines:

            app.post("/getFile", function (req,res){
            // some code...
              fs.read(fd, buffer, 0, byteRoRead, start, function(err, bytesRead, buffer){
                  buffer.pipe(res);
               }) 
             })

Because the client sent incorrect start param, my server failed with the next error:

fs.js:457
  binding.read(fd, buffer, offset, length, position, wrapper);
          ^
Error: Offset is out of bounds

So now I can fix it, but there is many other error that I can't predict, so I want to gives a client response of unexpected error, but I want that my server would still alive.

Answer 1

Express has built in error handling as a middleware. See http://expressjs.com/guide.html#error-handling for a more comprehensive guide but as an example:

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

This middleware would catch the error, log it to the console and return a 500 page to the user. This also will stop the server crashing. If you wanted to log the error, but still let the server crash add throw err; to the body of that function.

This is sometimes a good idea as you may not want the server to continue to serve requests if it is in some unexpected error state. Refer to http://shapeshed.com/uncaught-exceptions-in-node/ as a good brief guide on the subject. Also, see http://www.joyent.com/developers/node/design/errors for a more detailed discussion.