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event-emitter.js
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event-emitter.js
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// Design an EventEmitter class. This interface is similar (but with some differences)
// to the one found in Node.js or the Event Target interface of the DOM.
// The EventEmitter should allow for subscribing to events and emitting them.
// Your EventEmitter class should have the following two methods:
// 1. subscribe - This method takes in two arguments:
// the name of an event as a string and a callback function.
// This callback function will later be called when the event is emitted.
// An event should be able to have multiple listeners for the same event.
// When emitting an event with multiple callbacks,
// each should be called in the order in which they were subscribed.
// An array of results should be returned.
// You can assume no callbacks passed to subscribe are referentially identical.
// The subscribe method should also return an object with an unsubscribe method
// that enables the user to unsubscribe. When it is called,
// the callback should be removed from the list of subscriptions and undefined should be returned.
// 2. emit - This method takes in two arguments:
// the name of an event as a string and an optional array of arguments
// that will be passed to the callback(s).
// If there are no callbacks subscribed to the given event, return an empty array.
// Otherwise, return an array of the results of all callback calls in the order they were subscribed.
class EventEmitter {
constructor() {
this.events = {};
}
/**
* @param {string} eventName
* @param {Function} callback
* @return {Object}
*/
subscribe(eventName, callback) {
if (!this.events[eventName]) {
this.events[eventName] = [];
}
this.events[eventName].push(callback);
return {
unsubscribe: () => {
const cbIndex = this.events[eventName].indexOf(callback);
if (cbIndex !== -1) {
this.events[eventName].splice(cbIndex, 1);
}
}
};
}
/**
* @param {string} eventName
* @param {Array} args
* @return {Array}
*/
emit(eventName, args = []) {
if (!this.events[eventName]) {
return [];
}
return this.events[eventName].map(cbItem => cbItem(...args));
}
}
const emitter = new EventEmitter();
function onClickCallback() {
return 99;
}
const sub = emitter.subscribe('onClick', onClickCallback);
console.log(emitter.emit('onClick')); // [99]
console.log(sub.unsubscribe()); // undefined
console.log(emitter.emit('onClick')); // []