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

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();
    });

});