Domains not properly catching errors while testing nodeJS in mocha

Question

When running tests that utilize domains for error handling, Mocha still appears to be throwing an error even if a domain handler inside a library should have caught the erro

Answer 1

nodejs domains do definitely catch synchronous errors

See this simple test case

var domain = require("domain");
var d = domain.create();

d.on("error", function() {
    console.log("domain caught");
});


d.run(function() {
    throw new Error("foo");
});


// result: domain caught

EDIT: Since writing this answer I have written a blog post describing what is going on with domains and try catch and whether you can use domains as a wholesale replacement for try catch. It summarises most of what has been discussed here.

http://www.lighthouselogic.com/node-domains-as-a-replacement-for-try-catch/

ORIGINAL ANSWER:

Actually there is a problem with Mocha.

I wrote the following test function:

function error(callback){
     var d = domain.create().on('error', function(err){
        console.log("Caught Error in Domain Handler")
        return callback(err);
    });
    d.enter();
    throw new Error("TestError");
    d.exit();
}

Then I wrote a simple test without mocha:

error(function(err){
    if(err)
    {
        console.log("Error was returned");
    }else
    {
        console.log("Error was not returned")
    }
})

The output I received was:

Caught Error in Domain Handler
Error was returned

When I tested using Mocha:

describe('Domain Tests', function(){
    it('Should return an error when testing', function(done){
        error(function(err){
            if(err)
            {
                console.log("Error was returned");
            }else
            {
                console.log("Error was not returned")
            }
            return done();
        })
    });
});

I received the following output:

․

  0 passing (4ms)
  1 failing

  1) Domain Tests Should return an error when testing:
     Error: TestError
      at error (/Users/bensudbury/Documents/node_projects/testMochaDomains/test.js:9:11)
      at Context. (/Users/bensudbury/Documents/node_projects/testMochaDomains/testMocha.js:6:3)
      at Test.Runnable.run (/usr/local/share/npm/lib/node_modules/mocha/lib/runnable.js:194:15)
      at Runner.runTest (/usr/local/share/npm/lib/node_modules/mocha/lib/runner.js:358:10)
      at /usr/local/share/npm/lib/node_modules/mocha/lib/runner.js:404:12
      at next (/usr/local/share/npm/lib/node_modules/mocha/lib/runner.js:284:14)
      at /usr/local/share/npm/lib/node_modules/mocha/lib/runner.js:293:7
      at next (/usr/local/share/npm/lib/node_modules/mocha/lib/runner.js:237:23)
      at Object._onImmediate (/usr/local/share/npm/lib/node_modules/mocha/lib/runner.js:261:5)
      at processImmediate [as _immediateCallback] (timers.js:330:15)

As you can see: the domain error handler was short circuited.

This problem seems to be related to the following issues:

https://github.com/visionmedia/mocha/issues/513

While the Node issue has been closed, the issue in mocha is still open.

The workaround that was suggested at: https://gist.github.com/mcollina/4443963 didn't resolve the issue in this case.

I dug through the code of Mocha and found that the problem occurs because mocha wraps the tests in a try catch block. This means that the exception is caught and never sent to the uncaughtException or _fatalException handler depending upon the version of node that you are using.

Your workaround is good, but nodejs domains do definitely catch synchronous errors so I wouldn't change your code but instead change your test. Your new test should look like:

describe("test", function() {
    it("should succeed", function(done) {
        process.nextTick(function(){
            var foo = require("./foo.js");
            foo(function() {
                console.log("done");
                done();
            });
        })      
    });
});

I haven't tested this code, but similar code for my example works properly:

it('Should return an error when testing', function(done){   
    process.nextTick(function(){
        error(function(err){
            if(err)
            {
                console.log("Error was returned");
            }else
            {
                console.log("Error was not returned")
            }
            return done();
        });
    })
});

I have added a comment to the end of the issue in Mocha to see if it can be resolved:

https://github.com/visionmedia/mocha/issues/513