web-stream-map/index.js

A map function for Web Streams with concurrency support. Like node's ReadableStream.map(), but for Web Streams.

import sCompose from "s-compose";
import pLimit from "p-limit";

export default function sMap(fn, { concurrency = 1 } = {}) {
  const limit = pLimit(concurrency);

  return sCompose(
    new TransformStream(
      {
        async transform(chunk, controller) {
          const p = limit(() => fn(chunk));
          controller.enqueue(p);
        },
      },
      {},
      { highWaterMark: concurrency * 2 - 1 }
    ),
    new TransformStream({
      async transform(chunk, controller) {
        const result = await chunk;
        controller.enqueue(result);
      },
    })
  );
}