Atomic Update Problem

atomic.js

var stream = require('stream');
var fs = require('fs');

// clear the data file
fs.writeFileSync('data.txt', '0');

// Returns a stream that emits a series of asynchronously generated events
var getStream = function() {
  var s = new stream.Stream();

  var work = [];
  for (var i = 0; i < 10; i++) {
    work.push(i);
  }

  var next = function() {
    var val = work.shift();
    s.emit('data', val);

    if (work.length) {
      setTimeout(next, 0);
    } else {
      s.emit('end');
    }
  };

  setTimeout(next, 0);

  return s;
};

var s = getStream();
s.on('data', function(data) {
  // Ideally would like this read/write 'increment' update to be atomic
  fs.readFile('data.txt', function(err, fileData) {
    var val = parseInt(fileData.toString(), 10);
    val++;

    fs.writeFile('data.txt', val, function(err) {
      console.log(val + ' written to file');
    });
  });
});

// check the final written value in the file
// Ideally the final value written in the file would be 10
s.on('end', function() {
  var val = parseInt(fs.readFileSync('data.txt').toString());
  console.log('Final value = ' + val);
});

Currently the output is something like this (indeterminate each time you run it):

1 written to file
2 written to file
2 written to file
2 written to file
2 written to file
2 written to file
2 written to file
2 written to file
2 written to file
2 written to file