Stream-transform JSONL/NDJSON in Node.js with filter/map style processing, stdin to stdout, and proper backpressure handling (no deps).

node-transform-jsonl-stream.js

#!/usr/bin/env node
/**
 * Stream-transform JSONL (NDJSON) from stdin to stdout with backpressure.
 *
 * Features:
 * - Reads JSONL from stdin
 * - Parses one JSON object per line
 * - Applies transform/filter logic
 * - Writes JSONL to stdout
 * - Respects stdout backpressure
 * - No dependencies
 *
 * Usage:
 *   cat input.jsonl | node node-transform-jsonl-stream.js > output.jsonl
 *
 * Example:
 *   cat input.jsonl | node node-transform-jsonl-stream.js
 */

const readline = require("readline");
const { once } = require("events");

async function writeLine(line) {
  if (!process.stdout.write(line + "\n")) {
    await once(process.stdout, "drain");
  }
}

async function transformRecord(record) {
  // Example filter:
  // Skip records marked inactive
  if (record.active === false) {
    return null;
  }

  // Example map:
  return {
    ...record,
    processed_at: new Date().toISOString(),
  };
}

async function main() {
  const rl = readline.createInterface({
    input: process.stdin,
    crlfDelay: Infinity,
  });

  let lineNum = 0;

  try {
    for await (const line of rl) {
      lineNum += 1;

      // Skip blank lines
      if (!line.trim()) {
        continue;
      }

      let record;
      try {
        record = JSON.parse(line);
      } catch (err) {
        throw new Error(`Invalid JSON on line ${lineNum}: ${err.message}`);
      }

      const transformed = await transformRecord(record);

      // Null means "filtered out"
      if (transformed === null || transformed === undefined) {
        continue;
      }

      await writeLine(JSON.stringify(transformed));
    }
  } finally {
    rl.close();
  }
}

main().catch((err) => {
  console.error("ERROR:", err && err.stack ? err.stack : err);
  process.exit(1);
});

Optional example input:

{"id":1,"name":"Alice","active":true}
{"id":2,"name":"Bob","active":false}
{"id":3,"name":"Charlie","active":true}

Example output:

{"id":1,"name":"Alice","active":true,"processed_at":"2026-03-06T00:00:00.000Z"}
{"id":3,"name":"Charlie","active":true,"processed_at":"2026-03-06T00:00:00.000Z"}