dev-notes/Node.js/Standard Packages/events.md

Events Module

Much of the Node.js core API is built around an idiomatic asynchronous event-driven architecture in which certain kinds of objects (emitters) emit named events that cause Function objects (listeners) to be called.

All objects that emit events are instances of the EventEmitter class. These objects expose an eventEmitter.on() function that allows one or more functions to be attached to named events emitted by the object. Typically, event names are camel-cased strings but any valid JavaScript property key can be used.

When the EventEmitter object emits an event, all of the functions attached to that specific event are called synchronously. Any values returned by the called listeners are ignored and discarded.

const EventEmitter = require("events");

class CustomEmitter extends EventEmitter {} ;

const customEmitterObj = new CustomEmitter();

// add event listener
customEmitterObj.on("event", (e) => {
    // e contains event object
})

// single-use event listener (execute and remove listener)
customEmitterObj.once("event", (e) => {
    // e contains event object
})

customEmitterObj.removeListener("event", callback);
customEmitterObj.removeAllListeners("event");

customEmitterObj.emit("event");
customEmitterObj.emit("event", { /* event object */ });

customEmitterObj.eventNames();  // string[] of the events it listens to
customEmitterObj.listenerCount("event");  // num of listeners for an event
customEmitterObj.listeners();  // Function[], handlers of the events