How to create custom event listener in node.js (express.js)?

Question

I have a giant function with a lot of nested callbacks. I want to make it cleaner and easier to handle. So, I'm thinking of using custom event listeners

Like, when a function is done, in the callback, instead of putting a chunk of code, it just emits an event, and then the listener will run.

So, how to do that in node.js? I am still looking for a good example to guide me through.

Related http://stackoverflow.com/questions/4903154/how-do-i-avoid-deeply-nested-code-in-node-js? — Captain Giraffe, Apr 21 '11 at 02:12
I know it's an old question, but how about `async.js` instead of event listeners? — Sherzod, Oct 16 '14 at 17:48

score 29 · Answer 1 · edited May 07 '12 at 08:43

29

You can set events like this

app.on('event:user_created', callback);

Then you can emit them

app.emit('event:user_created', data);

express.js uses EventEmitter.

edited May 07 '12 at 08:43

bluish

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answered May 07 '12 at 05:32

StephaneP

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It's actually that `app` extends `EventEmitter` in **connect** framework which **express** extends . [connect.js@68](https://github.com/senchalabs/connect/blob/master/lib/connect.js#L68) `utils.merge(app, EventEmitter.prototype);` – antitoxic Oct 07 '12 at 17:05
So the express `app` can act as an app-wide dispatch – Dmitry Minkovsky Jun 26 '13 at 19:58
will this take an err object - app.emit('event:foo', err, data) - for instance if we're returning the result of a database query? – starsinmypockets Jan 07 '14 at 02:19
how would you call this from a route file? How do you get access to app? – Samuel Goldenbaum Dec 01 '14 at 14:56
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I don't think this works anymore with express.js 4. All I get when I call app.emit() is an undefined is not a function error – Luis Elizondo Oct 05 '15 at 13:38
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in Express.js you can do `req.app.emit(...)`. Because every request object has a reference of the app – danielrvt May 14 '20 at 11:30

score 9 · Accepted Answer · answered Apr 25 '11 at 08:39

9

You probably want to create an EventEmitter object, and call emit() on it.

answered Apr 25 '11 at 08:39

mjs

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score 6 · Answer 3 · edited Dec 29 '15 at 00:29

I just came across this question, I'd like to chip in my 2 cents, specifically responding to Luis.

In Express, if you want the 'app' instance to listen for a custom event you would do something like:

app.on('testEvent', function () {
  return console.log('responded to testEvent');
});

app.get('/test', function (req, res) {
  app.emit('testEvent');
  return res.status(200).end();
});

Then when that route is hit, you'd see the response in the console. Of course you can implement this as you wish.

score 1 · Answer 4 · answered Sep 18 '20 at 16:01

The Node.js "events" module and the "EventEmitter" module facilitates communication between objects in Node. The EventEmitter module is at the core of Node's asynchronous event-driven architecture. Here is how you can create custom events and emit them:

const EventEmitter = require('events');
const myEmitter = new EventEmitter();

//Event Listener
const EventListenerFunc = () => {
 console.log('an event occurred!');
}

//Registering the event with the listener
myEmitter.on('eventName', EventListenerFunc);

//Emitting the event 
myEmitter.emit('eventName');

How to create custom event listener in node.js (express.js)?

4 Answers4