Events and Streams¶

EventEmitter (events module)¶

Node.js uses the EventEmitter pattern to handle asynchronous events. It's part of the built-in events module.

const EventEmitter = require("events");
const emitter = new EventEmitter();

Emitting and Listening to Events¶

emitter.on("greet", (name) => {
  console.log(`Hello, ${name}`);
});

emitter.emit("greet", "Alice"); // Hello, Alice

Multiple Listeners¶

You can register multiple listeners to the same event.

emitter.on("log", () => console.log("Listener 1"));
emitter.on("log", () => console.log("Listener 2"));
emitter.emit("log");

Once¶

Use .once() to listen only one time.

emitter.once("data", () => console.log("Received once"));
emitter.emit("data"); // Triggers
emitter.emit("data"); // Ignored

Remove Listener¶

function log() {
  console.log("Log event");
}

emitter.on("log", log);
emitter.removeListener("log", log);

Streams¶

Streams handle data in chunks, great for large files or network traffic.

Four types:

Readable
Writable
Duplex (read + write)
Transform (modify during read/write)

Readable Stream Example¶

const fs = require("fs");
const readStream = fs.createReadStream("bigfile.txt", "utf8");

readStream.on("data", (chunk) => {
  console.log("Received chunk:", chunk);
});

Writable Stream Example¶

const writeStream = fs.createWriteStream("output.txt");

writeStream.write("Line 1\n");
writeStream.write("Line 2\n");
writeStream.end();

Pipe¶

Use .pipe() to connect readable to writable stream.

const fs = require("fs");

const input = fs.createReadStream("source.txt");
const output = fs.createWriteStream("copy.txt");

input.pipe(output);

Buffer¶

Buffers are raw binary data stored in memory outside the V8 heap.

const buf = Buffer.from("Hello");
console.log(buf);           // <Buffer 48 65 6c 6c 6f>
console.log(buf.toString()); // Hello

Use Buffers when working with streams, sockets, or binary files.

Summary¶

Concept	Description
EventEmitter	Emit/listen for custom async events
Stream	Handle large data in chunks
Pipe	Connect readable → writable streams
Buffer	Low-level binary data container