fs.watchFile() -> fs.read()

watch-and-read.js

var fs = require('fs');
var filename = process.argv[2];
fs.open(filename, 'r', function(err, fd) {
  fs.watchFile(filename, function(cstat, pstat) {
    var delta = cstat.size - pstat.size;
    if (delta <= 0) return;

    fs.read(fd, new Buffer(delta), 0, delta, pstat.size, function(err, bytes, buffer) {
      console.log("err", err, "delta", delta, "bytes", bytes, "buffer", buffer.toString());
    });
  });
});

If you want this to load data already in the file you will need to modify it:

var fs = require('fs');
var filename = process.argv[2];
fs.open(filename, 'r', function(err, fd) {
  if (err) throw new Error('Could not open file');
  var position = 0;
  fs.stat(filename, read);
  fs.watchFile(filename, read.bind(null, null));

  function read(err, stat) {
    var delta = stat.size - position;
    if (delta <= 0) return;

    fs.read(fd, new Buffer(delta), 0, delta, position, function(err, bytes, buffer) {
      console.log("err", err, "delta", delta, "bytes", bytes, "buffer", buffer.toString());
    });
    position = stat.size;
  } 
});

This is pretty rough code though, please be sure to handle errors. The file may be removed, truncated, etc.