How can I test uncaught errors in mocha?

Question

I would like to test that the following function performs as expected:

function throwNextTick(error) {
    process.nextTick(function () {
        throw error;
           


        

Answer 1

Base on timoxley & Casey Foster, in node v6++

const assert = require('assert')

describe('throwNextTick', function() {
    it('works as expected', function(next) {

        function cb(err) {
            assert.equal(err instanceof Error, true)
            next()
        }

        function test(){
            process.nextTick(() => {
                throw new Error('err')
            })
        }

        process.prependOnceListener('uncaughtException', cb)
        test()

    })
})

Answer 2

If your async code is executed within a domain - and that is often the case - you need to change the error listener on the domain instead of the process.

For that you can use:

it('should produce an unhandled exception', function (done) {

    // Remove Mocha's error listener
    var originalErrorListeners = process.domain.listeners('error');
    process.domain.removeAllListeners('error');

    // Add your own error listener to check for unhandled exceptions
    process.domain.on('error', function () {

        // Add the original error listeners again
        process.domain.removeAllListeners('error');
        for ( var i = 0; i < originalErrorListeners.length; i+=1 ) {
            process.domain.on('error', originalErrorListeners[i]);
        }

        // For the sake of simplicity we are done after catching the unhandled exception
        done();

    });

    // This would be your async application code you expect to throw an exception
    setTimeout(function () {
        throw new Error();
    });

});

Answer 3

Update: Courtesy of casey-foster in a comment below:

As of node v6.0.0 you can use process.prependOnceListener('uncaughtException', ...) to do this much more succinctly.

---

Old answer:

The secret lies in process.listeners('uncaughtException'):

http://nodejs.org/docs/latest/api/events.html#emitter.listeners

Simply remove the mocha listener, add your own, then reattach the mocha listener.

See below:

var assert = require('assert')

function throwNextTick(error) {
    process.nextTick(function () {
        throw error
    })
}


describe("throwNextTick", function () {
    it("works as expected", function (next) {
        var error = new Error("boo!")
        var recordedError = null
        var originalException = process.listeners('uncaughtException').pop()
        //Needed in node 0.10.5+
        process.removeListener('uncaughtException', originalException);
        process.once("uncaughtException", function (error) {
            recordedError = error
        })
        throwNextTick(error);
        process.nextTick(function () {
            process.listeners('uncaughtException').push(originalException)
            assert.equal(recordedError, error)
            next()
        })
    })
})