first-chunk-stream

What is first-chunk-stream?

The 'first-chunk-stream' npm package is a Node.js module that allows you to transform the first chunk of a stream. This can be particularly useful for tasks that require modifying or inspecting the beginning of a stream before processing the rest, such as adjusting headers, parsing initial metadata, or prepending data to a stream.

What are first-chunk-stream's main functionalities?

Modify the first chunk of a stream

This code sample demonstrates how to use first-chunk-stream to modify the first 10 bytes of data from a file stream. It prepends the text 'Modified: ' to the beginning of the first chunk.

const firstChunkStream = require('first-chunk-stream');
const fs = require('fs');

const modifyFirstChunk = firstChunkStream({ chunkSize: 10 }, (chunk, enc, callback) => {
  const modifiedChunk = Buffer.from(`Modified: ${chunk.toString()}`);
  callback(null, modifiedChunk);
});

fs.createReadStream('example.txt')
  .pipe(modifyFirstChunk)
  .pipe(fs.createWriteStream('modifiedExample.txt'));

Inspect and conditionally modify the first chunk

This example shows how to inspect the first 15 bytes of a stream and modify it only if it contains the word 'Important'. If the condition is met, it prepends 'Alert: ' to the chunk.

const firstChunkStream = require('first-chunk-stream');
const fs = require('fs');

const inspectFirstChunk = firstChunkStream({ chunkSize: 15 }, (chunk, enc, callback) => {
  if (chunk.toString().includes('Important')) {
    callback(null, Buffer.from(`Alert: ${chunk.toString()}`));
  } else {
    callback(null, chunk);
  }
});

fs.createReadStream('example.txt')
  .pipe(inspectFirstChunk)
  .pipe(fs.createWriteStream('inspectedExample.txt'));

Other packages similar to first-chunk-stream

through2

Through2 is a tiny wrapper around Node streams.Transform, making it easier to create transform streams. It is similar to first-chunk-stream but offers more general functionality for transforming streams, not limited to just the first chunk.

peek-stream

Peek-stream allows you to peek at the first few bytes of a stream to determine how to parse it, then lets the stream continue unmodified or modified. It is similar to first-chunk-stream but focuses on peeking rather than modifying.

README

first-chunk-stream

Buffer and transform the n first bytes of a stream

Install

npm install first-chunk-stream

Usage

import fs from 'node:fs';
import getStream from 'get-stream';
import FirstChunkStream from 'first-chunk-stream';
import {uint8ArrayToString} from 'uint8array-extras';

// unicorn.txt => unicorn rainbow
const stream = fs.createReadStream('unicorn.txt')
	.pipe(new FirstChunkStream({chunkSize: 7}, async (chunk, encoding) => {
		return uint8ArrayToString(chunk).toUpperCase();
	}));

const data = await getStream(stream);

if (data.length < 7) {
	throw new Error('Couldn\'t get the minimum required first chunk length');
}

console.log(data);
//=> 'UNICORN rainbow'

API

FirstChunkStream(options, transform)

FirstChunkStream constructor.

transform(chunk, encoding)

Type: Function

Async function that receives the required options.chunkSize bytes.

Expected to return an buffer-like object or string or object of form {buffer: Uint8Array, encoding: string} to send to stream or firstChunkStream.stop to end stream right away.

An error thrown from this function will be emitted as stream errors.

Note that the buffer can have a smaller length than the required one. In that case, it will be due to the fact that the complete stream contents has a length less than the options.chunkSize value. You should check for this yourself if you strictly depend on the length.

import FirstChunkStream from 'first-chunk-stream';

new FirstChunkStream({chunkSize: 7}, async (chunk, encoding) => {
	return chunk.toString(encoding).toUpperCase(); // Send string to stream
});

new FirstChunkStream({chunkSize: 7}, async (chunk, encoding) => {
	return chunk; // Send buffer to stream
});

new FirstChunkStream({chunkSize: 7}, async (chunk, encoding) => {
	return {
		buffer: chunk,
		encoding: encoding,
	}; // Send buffer with encoding to stream
});

new FirstChunkStream({chunkSize: 7}, async (chunk, encoding) => {
	return FirstChunkStream.stop; // End the stream early
});

new FirstChunkStream({chunkSize: 7}, async (chunk, encoding) => {
	throw new Error('Unconditional error'); // Emit stream error
});

options

Type: object

The options object is passed to the Duplex stream constructor allowing you to customize your stream behavior. In addition, you can specify the following option:

chunkSize

Type: number

The number of bytes to buffer.