Node.js Events

Many Node objects are event emitters: they announce things with emit, and you react with on.

ExampleNode.js

const EventEmitter = require("events");

class Chat extends EventEmitter {}
const chat = new Chat();

chat.on("message", (user, text) => console.log(`${user}: ${text}`));
chat.on("message", (user) => console.log(`(logged a message from ${user})`));
chat.once("join", (user) => console.log(`${user} joined`));

chat.emit("join", "Ada");
chat.emit("join", "Sam");       // once: no output
chat.emit("message", "Ada", "Hello!");
console.log(chat.listenerCount("message"));

Output

Ada joined
Ada: Hello!
(logged a message from Ada)
2

Quick check

Which method triggers an event?

emittriggerfire

Finish the quick check (and any code exercise) to complete this lesson, or mark it done yourself.

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