Why isn't this EventEmitter pubsub singleton interface working in node.js? - node.js

Question

I am trying to separate publisher and subscriber in node.js to be able to send data to each other through a shared EventEmitter instance as bus.
My bus follows the singleton method discussed [HERE][1]
bus.js file
// https://derickbailey.com/2016/03/09/creating-a-true-singleton-in-node-js-with-es6-symbols/
// create a unique, global symbol name
// -----------------------------------
const FOO_KEY = Symbol.for("test.exchanges.bus");
const EventEmitter = require("events");
// check if the global object has this symbol
// add it if it does not have the symbol, yet
// ------------------------------------------
var globalSymbols = Object.getOwnPropertySymbols(global);
var hasFoo = (globalSymbols.indexOf(FOO_KEY) > -1);
if (!hasFoo){
global[FOO_KEY] = {
foo: new EventEmitter()
};
}
// define the singleton API
// ------------------------
var singleton = {};
Object.defineProperty(singleton, "instance", {
get: function(){
return global[FOO_KEY];
}
});
// ensure the API is never changed
// -------------------------------
Object.freeze(singleton);
// export the singleton API only
// -----------------------------
module.exports = singleton;
My understanding is that when I require this file in different modules, the same foo Object should be made available. Isn't that the purpose of having a singleton?
pub.js file
const bus = require("./bus");
class Publisher {
constructor(emitter) {
this.emitter = emitter;
console.log(this.emitter);
this.test();
}
test() {
setInterval(() => {
this.emitter.emit("test", Date.now());
}, 1000);
}
}
module.exports = Publisher;
console.log(bus.instance.foo);
sub.js file
const bus = require("./bus");
class Subscriber {
constructor(emitter) {
this.emitter = emitter;
console.log(this.emitter);
this.emitter.on("test", this.handleTest);
}
handleTest(data) {
console.log("handling test", data);
}
}
module.exports = Subscriber;
console.log(bus.instance.foo);
When I run pub.js and sub.js on 2 separate terminal windows, sub.js finished executing immediately as if publisher is not pushing the messages to it. Could anyone kindly point how to separate the publisher and subscriber to work with the same event bus?

Answer 1

You may consider re-designing your bus module. I recommend creating it as a class which extends EventEmitter and then returning an instantiated instance of this class.
Now when this file is loaded the first time, the class code will run and an object will be instantiated and then exported back. require will cache this instance and next time when this file is loaded, it will get the same object back. this makes it a singleton and you can now use this as a common bus.
here is some code to demonstrate this point:
bus.js
const {EventEmitter} = require('events');
class Bus extends EventEmitter {
constructor(){
super();
setTimeout(() => this.emit('foo', (new Date()).getTime()), 1000);
}
}
module.exports = new Bus();
bus.spec.js
const bus1 = require('../src/util/bus');
const bus2 = require('../src/util/bus');
describe('bus', () => {
it('is the same object', () => {
expect(bus1).toEqual(bus2);
expect(bus1 === bus2).toEqual(true);
});
it('calls the event handler on bus2 when event is emitted on bus1', done => {
bus2.on('chocolate', flavor => {
expect(flavor).toBe('dark');
done()
});
bus1.emit('chocolate', 'dark');
}, 20)
});