Node.js: What are the differences between Readable, Writable, Transform, and Duplex streams?

Readable — produces data. Writable — consumes data. Duplex — both. Transform — modifies data in between.

Readable:

• fs.createReadStream, http.IncomingMessage.
• Events: data, end, error.
• Methods: read(), pipe().

Writable:

• fs.createWriteStream, http.ServerResponse.
• Events: drain, finish, error.
• Methods: write(), end().

Duplex:

• net.Socket.
• Both readable and writable.
• Independent read/write sides.

Transform:

• crypto.createCipher, zlib.createGzip.
• Readable + Writable.
• Data is modified in the transform() method.

1const { Transform } = require("stream");
2const upperCase = new Transform({
3    transform(chunk, encoding, callback) {
4        callback(null, chunk.toString().toUpperCase());
5    }
6});

Key difference:

• Transform = Duplex where output depends on input.