emit error event is used to return an error which occurs during the execution of the component.

Here is an example implementation:

function emitError(err) {
    this.emit('error', err);

This is one of the simplistic implementations of the emit error event. For more advanced usage one could consult the following example from elastic.io Keen.io component:

var Keen = require('keen-js');
var Q = require('q');
var elasticio = require('elasticio-node');
var messages = elasticio.messages;

exports.process = execute;

function execute(msg, cfg) {

    var that = this;


    function validateConfig(cfg) {

        if (!cfg.writeKey) {
            throw new Error('writeKey is required to perform the sendEvent action, please reconfigure your account.');

        if (!cfg.projectId) {
            throw new Error('Please provide a valid projectId.');

        if (!cfg.eventCollection) {
            throw new Error('Please provide a Collection Name for your events.');

        return cfg;

    function createClient(cfg) {
        return new Keen({
            writeKey: cfg.writeKey,
            projectId: cfg.projectId

    function sendEvent(client) {
        return Q.ninvoke(client, 'addEvent', cfg.eventCollection, msg.body);

    function emitSuccess(response) {
        var newMsg = messages.newMessageWithBody(response);

        that.emit('data', newMsg);

    function emitError(err) {
        that.emit('error', err);

    function end() {

Lines 55-57 show the same structure of emit error, however, this time we see how the error event is going to be called. It will be called when any of the functions listed on Lines 13-16 fail. It is important to note that this implementation method is made possible by use of promises via q-promises.