What are Streams in Node.js? | Node.js Interview Questions

What You'll Learn

What streams are
Types of streams
When to use streams

What are Streams?

Streams are objects that let you read/write data piece by piece, without loading entire data into memory.

Types of Streams

Type	Description	Example
Readable	Read data from source	fs.createReadStream()
Writable	Write data to destination	fs.createWriteStream()
Duplex	Both read and write	net.Socket
Transform	Modify data while reading/writing	zlib.createGzip()

Example - Reading Large File

code.jsJavaScript

const fs = require('fs');

// ❌ Bad - loads entire file into memory
const data = fs.readFileSync('large-file.txt');

// ✅ Good - processes in chunks
const stream = fs.createReadStream('large-file.txt');
stream.on('data', (chunk) => {
  console.log('Received chunk:', chunk.length);
});
stream.on('end', () => console.log('Done'));

Piping Streams

code.jsJavaScript

const fs = require('fs');
const zlib = require('zlib');

// Compress file using pipe
fs.createReadStream('input.txt')
  .pipe(zlib.createGzip())
  .pipe(fs.createWriteStream('input.txt.gz'));