Exceptions thrown in asynchronous javascript not caught

Question

Exceptions thrown in asynchronous javascript not caught

Basically, why isn't this exception caught?

var http = require('http'),
    options = {
      host: 'www.crash-boom-bang-please.com',
      port: 80,
      method: 'GET'
    };

try {
  var req = http.request(options, function(res) {
    res.setEncoding('utf8');
    res.on('data', function (chunk) {
      console.log('BODY: ' + chunk);
    });
  });

  req.on('error', function(e) {
    throw new Error("Oh noes");
  });
  req.end();
} catch(_error) {
  console.log("Caught the error");
}

Some people suggest that these errors need to be handled with event emitters or callback(err) (having callbacks with err, data signature isn't something I'm used to)

What's the best way to go about it?

javascript
node.js
asynchronous
exception-handling

Answer 1

When you are throwing the error the try {} block has been long left, as the callback is invoked asynchronously outside of the try/catch. So you cannot catch it.

Do whatever you want to do in case of an error inside the error callback function.

Answer 2

As of node version 0.8 you can constrain exceptions to domains. You can constrain exceptions to a certain domain and catch them at that scope

If you're interested, I've written a small function that catches asynchronous exceptions here: Javascript Asynchronous Exception Handling with node.js. I'd love some feedback This will let you perform the following:

var http = require('http'),
    options = {
      host: 'www.crash-boom-bang-please.com',
      port: 80,
      method: 'GET'
    };

atry(function(){
  var req = http.request(options, function(res) {
    res.setEncoding('utf8');
    res.on('data', function (chunk) {
      console.log('BODY: ' + chunk);
    });
  });

  req.on('error', function(e) {
    throw new Error("Oh noes");
  });
  req.end();
}).catch(function(_error) {
  console.log("Caught the error");
});