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

const fs = require('fs');
const getStream = require('get-stream');
const FirstChunkStream = require('first-chunk-stream');

// unicorn.txt => unicorn rainbow
const stream = fs.createReadStream('unicorn.txt')
	.pipe(new FirstChunkStream({chunkLength: 7}, (error, chunk, encoding, callback) => {
		if (error) {
			callback(error);
			return;
		}

		callback(null, chunk.toString(encoding).toUpperCase());
	}));

(async () => {
	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)

Returns a FirstChunkStream instance.

transform(error, chunk, encoding, callback)

Type: Function

The function that gets the required options.chunkLength bytes.

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.chunkLength value. You should check for this yourself if you strictly depend on the length.

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:

chunkLength

Type: number

How many bytes you want to buffer.

License

MIT © Sindre Sorhus