write.js

var fs = require("fs");
var path = require("path");
var outfile = path.resolve(__dirname, "appPrefs.json");
var lockfile = outfile + ".lock";

var data = "some data or other";

// obtain an exclusive read/write file handle on the semaphore file
fs.open(lockfile, "wx+", function (err, fd) {
  if (err) {
    console.warn("Failed to obtain exclusive lock");
  }
  else {
    // if we obtained the lock, we may write to the output file
    fs.writeFile(outfile, data, function (err) {
      if (err) {
        console.error(err.stack);
      }
      // close the file handle so we don't leak resources
      fs.close(fd, function (err) {
        if (err) {
          console.error(err.stack);
        }
        // remove the semaphore file
        fs.unlink(lockfile, function (err) {
          if (err) {
            console.error(err.stack);
            process.exit(1);
          }
          process.exit();
        });
      });
    });
  }
});