Readable Streams

TL;DR

A Readable stream represents a source from which data is consumed. It emits data in chunks rather than serving the whole payload at once.

Mental Model

How It Works

A Readable stream can operate in two modes:

1. Flowing Mode: Data is read from the underlying system automatically and provided to an application as quickly as possible via 'data' events.
2. Paused Mode: You must explicitly call stream.read() to get chunks of data out.

By default, all Readable streams start in paused mode. Attaching a 'data' event listener or using .pipe() switches them to flowing mode.

Example

const fs = require('fs');
const readable = fs.createReadStream('large-file.txt', { encoding: 'utf8' });

// Attaching the 'data' listener switches it to flowing mode
readable.on('data', (chunk) => {
    console.log(`Received ${chunk.length} bytes of data.`);
});

readable.on('end', () => {
    console.log('Finished reading file.');
});

Common Interview Questions

What happens when you call stream.pause()?

Calling stream.pause() stops the stream from emitting 'data' events, switching it from flowing mode back to paused mode. Data may still be buffered internally up to the highWaterMark.

Can you implement a custom Readable stream?

Yes, by importing Readable from node:stream and creating a class that extends it. You must implement the _read(size) method, where you push data using this.push(data). When done, push null.