through2

What is through2?

The through2 npm package is a thin wrapper around Node.js streams.Transform (a standard Node.js API) that makes it easier to create transform streams. It is designed to work with streams in object mode or non-object mode, allowing for simple function-based stream transformation. This package is particularly useful when you need to create a custom stream that modifies or transforms data as it passes through the stream pipeline.

What are through2's main functionalities?

Simple Transform

This feature allows you to create a simple transform stream that converts input data to uppercase. The code sample demonstrates how to use through2 to create a stream that reads from stdin, transforms the data, and writes to stdout.

const through2 = require('through2');
const stream = through2(function(chunk, enc, callback) {
  this.push(chunk.toString().toUpperCase());
  callback();
});
process.stdin.pipe(stream).pipe(process.stdout);

Object Mode Transform

This feature allows you to work with streams in object mode, where the stream chunks are JavaScript objects. The code sample shows how to create a transform stream that modifies properties of objects and how to use it with a readable stream of objects.

const through2 = require('through2').obj;
const stream = through2(function(obj, enc, callback) {
  obj.key = obj.key.toUpperCase();
  this.push(obj);
  callback();
});

// Usage with an array of objects
const objects = [{ key: 'value' }, { key: 'data' }];
const objectStream = require('stream').Readable.from(objects);
objectStream.pipe(stream).on('data', (transformedObj) => console.log(transformedObj));

Flush Function

This feature allows you to perform an action when the stream is ending, just before it finishes. The code sample demonstrates how to append a message to the stream output right before the stream ends.

const through2 = require('through2');
const stream = through2(function(chunk, enc, callback) {
  this.push(chunk);
  callback();
}, function(callback) {
  // This function is called before the stream is finished
  this.push('Stream is ending!');
  callback();
});

process.stdin.pipe(stream).pipe(process.stdout);

Other packages similar to through2

through

The 'through' package provides a simple way to create a readable-writable stream. It is similar to through2 but does not support object mode by default and has a less modern API.

stream-transform

The 'stream-transform' package is part of the Node.js CSV project and provides a standard transform stream with a simpler API for synchronous operations. It is specifically designed for CSV data transformation and is less generic compared to through2.

pumpify

The 'pumpify' package combines an array of streams into a single duplex stream using pump and duplexify. It is similar to through2 in that it deals with stream transformation but focuses on combining streams rather than creating individual transform streams.

README

The README file is missing