MongoDB/BSON ObjectId generator in Node.js

objectId.js

var os = require('os');
var crypto = require('crypto');

var machineId = crypto.createHash('md5').update(os.hostname()).digest().slice(0, 3);
var objectIdCounter = crypto.randomBytes(4).readUInt32BE() & 0xffffff;

// Returns a unique objectId as a hex string.
function newObjectId() {
  var buf = new Buffer(12);
  // Current time, 4 bytes.
  buf.writeUInt32BE(Math.floor(Date.now() / 1000), 0); 
  // Machine ID, 3 bytes.
  machineId.copy(buf, 4);
  // Process ID, 2 bytes.
  buf.writeUInt16BE(process.pid, 7);
  // Global counter, 3 bytes.
  buf.writeUInt8((objectIdCounter >>> 16) & 0xff, 9);
  buf.writeUInt8((objectIdCounter >>> 8) & 0xff, 10);
  buf.writeUInt8((objectIdCounter >>> 0) & 0xff, 11);
  objectIdCounter = (objectIdCounter + 1) & 0xffffff;
  return buf.toString('hex');
}