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

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()
        })
    })
})