express - How to handle code exceptions in node.js?

Question

I went through the documentation of Express, and the part describing error handling is completely opaque to me.

I figured the app they're referring to is an instance createServer(), right? But I have no clue how to stop node.js from blowing up the application process when an exception occurs during handling a request.

I don't need anything fancy really; I just want to return a status of 500, plus an otherwise empty response, whenever there's an exception. The node process must not terminate just because there was an uncaught exception somewhere.

Is there a simple example of how to achieve this?

---

var express = require('express');
var http = require('http');

var app = express.createServer();

app.get('/', function(req, res){
    console.log("debug", "calling")
    var options = {
        host: 'www.google.com',
        port: 80,
        path: "/"
    };
    http.get(options, function(response) {
       response.on("data", function(chunk) {
           console.log("data: " + chunk);
           chunk.call(); // no such method; throws here
       });
    }).on('error', function(e) {
       console.log("error connecting" + e.message);
    });
});

app.configure(function(){
    app.use(express.errorHandler({ dumpExceptions: true, showStack: true }));
});

app.listen(3000);

crashes the entire app, producing traceback

mypath/tst.js:16
           chunk.call(); // no such method; throws here
                 ^ TypeError: Object ... has no method 'call'
    at IncomingMessage.<anonymous> (/Library/WebServer/Documents/discovery/tst.js:16:18)
    at IncomingMessage.emit (events.js:67:17)
    at HTTPParser.onBody (http.js:115:23)
    at Socket.ondata (http.js:1150:24)
    at TCP.onread (net.js:374:27)

Answer 1

If you really want to catch all exceptions and provide some handling other than exiting the Node.js process, you need to handle Node's uncaughtException event.

If you think about it, this is a Node thing, and not an Express thing, because if you throw an exception from some arbitrary piece of code, there's no guarantee Express can or will ever see it, or be in a position to trap it. (Why? Exceptions don't interact very well with asynchronous event-driven callbacky code that is the Node style. Exceptions travel up the call stack to find a catch() block that's in scope at the time the exception is thrown. If myFunction defers some work to a callback function that runs when some event happens, then return to the event loop, then when that callback function is invoked, it's invoked directly from the main event loop, and myFunction is no longer on the call stack; if this callback function throws an exception, even if myFunction has a try/catch block, it's not going to catch the exception.)

What this means in practice is that if you throw an exception and don't catch it yourself and you do so in a function that was directly called by Express, Express can catch the exception and call the error handler you've installed, assuming you've configured some piece of error-handling middleware like app.use(express.errorHandler()). But if you throw the same exception in a function that was called in response to an asynchronous event, Express won't be able to catch it. (The only way it could catch it is by listening for the global Node uncaughtException event, which would be a bad idea first because that's global and you might need to use it for other things, and second because Express will have no idea what request was associated with the exception.)

Here's an example. I add this snippet of route-handling code to an existing Express app:

app.get('/fail/sync', function(req, res) {
   throw new Error('whoops');
});
app.get('/fail/async', function(req, res) {
   process.nextTick(function() {
      throw new Error('whoops');
   });
});

Now if I visit http://localhost:3000/fail/sync in my browser, the browser dumps a call stack (showing express.errorHandler in action). If I visit http://localhost:3000/fail/async in my browser, however, the browser gets angry (Chrome shows a "No data received: Error 324, net::ERR_EMPTY_RESPONSE: The server closed the connection without sending any data" message), because the Node process has exited, showing a backtrace on stdout in the terminal where I invoked it.