Concurrently write one JSON file from multiple processes spawned by the Node.js "cluster" module. Uses a lockfile to prevent hickups.

writeConcurrent.js

const fs = require('fs');
const os = require('os');
const path = require('path');
const cluster = require('cluster');

const lockFile = require('lockfile');

const filePath = path.resolve(__dirname, 'result.json');
const lockPath = path.resolve(__dirname, 'result.json.lock');

const lockOptions = {
  wait: 1000,
  stale: 5000,
  retries: 100,
  retryWait: 1000
};

(function init() {

  if (cluster.isMaster) {

    // Create empty JSON file or overwrite old JSON file
    fs.writeFileSync(filePath, JSON.stringify([]), 'utf8');

    // Create new worker for each CPU core
    for (var i = 0; i < os.cpus().length; i++) {

      cluster.fork().on('error', (error) => { console.error(error); });
    }
  } else if (cluster.isWorker) {

    writeToJson(cluster.worker.id);
  }
})();

function writeToJson(workerId) {

  lockFile.lock(lockPath, lockOptions, (error) => {

    if (error) { console.error(error); }

    // Open existing JSON file
    const oldJson = JSON.parse(fs.readFileSync(filePath, 'utf8'));
    // Merge object array with new data
    const newJson = oldJson.concat([{ id: workerId }]);

    console.log(newJson);

    // Save new data to the existing JSON file
    fs.writeFileSync(filePath, JSON.stringify(newJson), 'utf8');

    lockFile.unlock(lockPath, (error) => {

      if (error) { console.error(error); }

      cluster.worker.kill();
      process.exit(0);
    });
  });
}