1/** 2 * @fileoverview A variant of EventEmitter which does not give listeners information about each other 3 * @author Teddy Katz 4 */ 5 6"use strict"; 7 8//------------------------------------------------------------------------------ 9// Typedefs 10//------------------------------------------------------------------------------ 11 12/** 13 * An event emitter 14 * @typedef {Object} SafeEmitter 15 * @property {function(eventName: string, listenerFunc: Function): void} on Adds a listener for a given event name 16 * @property {function(eventName: string, arg1?: any, arg2?: any, arg3?: any)} emit Emits an event with a given name. 17 * This calls all the listeners that were listening for that name, with `arg1`, `arg2`, and `arg3` as arguments. 18 * @property {function(): string[]} eventNames Gets the list of event names that have registered listeners. 19 */ 20 21/** 22 * Creates an object which can listen for and emit events. 23 * This is similar to the EventEmitter API in Node's standard library, but it has a few differences. 24 * The goal is to allow multiple modules to attach arbitrary listeners to the same emitter, without 25 * letting the modules know about each other at all. 26 * 1. It has no special keys like `error` and `newListener`, which would allow modules to detect when 27 * another module throws an error or registers a listener. 28 * 2. It calls listener functions without any `this` value. (`EventEmitter` calls listeners with a 29 * `this` value of the emitter instance, which would give listeners access to other listeners.) 30 * @returns {SafeEmitter} An emitter 31 */ 32module.exports = () => { 33 const listeners = Object.create(null); 34 35 return Object.freeze({ 36 on(eventName, listener) { 37 if (eventName in listeners) { 38 listeners[eventName].push(listener); 39 } else { 40 listeners[eventName] = [listener]; 41 } 42 }, 43 emit(eventName, ...args) { 44 if (eventName in listeners) { 45 listeners[eventName].forEach(listener => listener(...args)); 46 } 47 }, 48 eventNames() { 49 return Object.keys(listeners); 50 } 51 }); 52}; 53