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

Transform the first chunk in a stream

Useful if you want to do something to the first chunk.

You can also set the minimum size of that chunk.

Install

$ npm install --save first-chunk-stream

Usage

var fs = require('fs');
var concat = require('concat-stream');
var firstChunk = require('first-chunk-stream');

// unicorn.txt => unicorn rainbow
// `highWaterMark: 1` means it will only read 1 byte at the time
fs.createReadStream('unicorn.txt', {highWaterMark: 1})
	.pipe(firstChunk({minSize: 7}, function (chunk, enc, cb) {
		this.push(chunk.toUpperCase());
		cb();
	}))
	.pipe(concat(function (data) {
		console.log(data);
		//=> UNICORN rainbow
	}));

API

firstChunk([options], transform)

options.minSize

Type: number

The minimum size of the first chunk.

transform(chunk, encoding, callback)

Required
Type: function

The function that gets the first chunk.

firstChunk.ctor()

Instead of returning a stream.Transform instance, firstChunk.ctor() returns a constructor for a custom Transform. This is useful when you want to use the same transform logic in multiple instances.

License

MIT © Sindre Sorhus