eslint/lib/linter/safe-emitter.js

   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 {(eventName: string, listenerFunc: Function) => void} on Adds a listener for a given event name
  16  * @property {(eventName: string, arg1?: any, arg2?: any, arg3?: any) => void} 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  */
  32 module.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 };