stream: add SyncTransform

[mcollina]

Summary

Add SyncTransform, a high-performance synchronous transform stream based on the syncthrough npm package.

Unlike the standard Transform stream, SyncTransform uses a synchronous transformation function (no callback), which results in significantly better performance (~10x faster).

Key Features

• Synchronous transform function for better performance
• Strict backpressure enforcement without internal buffering
• Works with pipeline(), pipe(), and 'data' events
• Supports object mode automatically
• Includes flush function for final data emission

Example

const { SyncTransform, pipeline } = require('node:stream');
const { createReadStream } = require('node:fs');

pipeline(
  createReadStream('input.txt'),
  SyncTransform((chunk) => chunk.toString().toUpperCase()),
  process.stdout,
  (err) => {
    if (err) console.error(err);
  }
);

Caveats

1. Strict backpressure: Writing before piping buffers exactly one chunk
2. Single destination: Only one pipe() destination allowed
3. No readable event: Data pushed directly to destination
4. Either pipe() or 'data': Cannot use both simultaneously
5. Object mode by default: Handles objects without explicit configuration

[nodejs-github-bot]

Review requested:

• @nodejs/streams

[ronag]

Awesome! But I have opinions... it's unfortunate that it tries to pretend to be a node stream even when it's clearly not.

I would prefer if it instead in some way was a construct for pipeline/compose. Something like:

pipeline(src, pipeline.map(chunk => chunk), dst, () => {})

The implementation could also be simpler then.