Events

events work differently than in browser
events are triggered manually, no browser window that magically triggers events through interaction
there are no built-in events, everything is created manually
there are no event objects, can pass arguments to EH when triggering event, event is just event name
event targets are called event emitters, more accurate name
there is no event flow

Event emitters

EM objects are instances of EventEmitter
eventEmitter.attachListener()/on(): adds EH to EM for event name, returns EM so calls can be chained
eventEmitter.removeListener()/off(): removes EH from EM for event name, returns EM so calls can be chained
eventEmitter.once(): adds one-time EH to EM for event name, returns EM so calls can be chained, i.e. like on() for an EH which calls off() inside itself
eventEmitter.emit(): calls EHs for event name synchronously, in order of registration, passing arguments to each, returns boolean if any EHs are attached

const EventEmitter = require("events");

class MyEmitter extends EventEmitter {}

const myEmitter = new MyEmitter();

myEmitter.on("something", () => {
    console.log("Reacting to event...");
});

myEmitter.on("error", err => {
    console.error("Whoops, an error occurred!", err);
});

myEmitter.emit("something");
// Reacting to event...

myEmitter.emit("error", new Error("Illegal operation."));
// Whoops, an error occurred! Error: Illegal operation.

Event handlers

EHs for same event are called sync and in order of attachment, must be non-async functions ❗️
this inside EH refers to EM which EH is registered on
return value of EH is ignored
can attach at most 10 EHs for given event by default, more will output a trace warning to stderr, can change using eventEmitter.setMaxListeners()
if EH is attached multiple times for same event and EM, it is called multiple times, needs to be removed multiple times as well ❗️
on "error" event, if no EH is attached, the first argument is thrown, a stack trace is printed, and the Node.js process exits, therefore always attach an error handler ❗️
removing EHs won’t stop calls for already emitted events

Event emitters

Event handlers

Resources