[JS] A Simple EventBus

eventBus.js

class EventBus {
  constructor() {
    this.eventMap = {}
    this.onceEventMap = {}
  }

  on(eventName, listener) {
    if (!this.eventMap[eventName]) {
      this.eventMap[eventName] = [listener]
    } else {
      this.eventMap[eventName].push(listener)
    }
  }
  emit(eventName, ...args) {
    const listeners = this.eventMap[eventName] || []
    listeners.forEach((listener) => listener.apply(null, args))
    if (this.onceEventMap[eventName]) {
      this.off(eventName, this.onceEventMap[eventName])
    }
  }
  off(eventName, listener) {
    if (!this.eventMap[eventName]) {
      return
    }
    if (this.eventMap[eventName].length === 1 || !listener) {
      delete this.eventMap[eventName]
      return
    }
    this.eventMap[eventName] = this.eventMap[eventName].filter(
      (item) => item !== listener
    )
  }
  once(eventName, listener) {
    this.on(eventName, listener)
    this.onceEventMap[eventName] = listener
  }
}