node.js NDJSON stream decoder

naive-ndjson.js

import { Transform } from "node:stream";
import { StringDecoder }  from "node:string_decoder";

const EOL = "\n",
      EOLLength = EOL.length;

export function createParser (encoding = "utf8") {
  return new Transform({
    decodeStrings: false,
    readableObjectMode: true,
    construct (cb) {
      this.bufferedData = "";
      this._decoder = new StringDecoder(encoding);

      cb();
    },
    transform (chunk, encoding, cb) {
      if (encoding === "buffer") {
        chunk = this._decoder.write(chunk);
      }
    
      for (let lastEOLIndex = -1;;) {
        const currEOLIndex = chunk.indexOf(EOL, lastEOLIndex);

        if (currEOLIndex === -1) {
          this.bufferedData += chunk;
          break;
        }

        try {
          const str = this.bufferedData + chunk.slice(lastEOLIndex, currEOLIndex),
                data = JSON.parse(str);

          this.push(data);

          this.bufferedData = "";
        } catch (err) {
          cb(err);
          return;
        }
        lastEOLIndex = currEOLIndex + EOLLength;
      }

      cb();
    },
    final (cb) {
      this.bufferedData += this._decoder.end();

      if (this.bufferedData.trim() === "") {
        cb();
        return;
      }

      try {
        this.push(JSON.parse(this.bufferedData));
        cb();
      } catch (err) {
        cb(err);
      }
    }
  });
}

// Use
const parser = createParser();

for await (const data of readableStream.pipe(parser)) {
  console.log(data);
}