Add JUUID to every Mongo document that doesn't contain one

uuidToMongoCollection.js

// Guid generator function based on stackoverflow
// http://stackoverflow.com/a/2117523/1282124
function guid(){
  return 'xxxxxxxx-xxxx-4xxx-yxxx-xxxxxxxxxxxx'.replace(/[xy]/g, function(c) {
    var r = Math.random()*16|0, v = c == 'x' ? r : (r&0x3|0x8);
    return v.toString(16);
  });
}

// Get a collection and add a uuid to each document 
// in that collection doesn't have one
function addUUIDToCollection(collectionName) {
  var col = db.getCollection(collectionName);
  col.find({ uuid: { $exists: false } }).forEach( function (doc) { doc["uuid"] = JUUID(guid()); col.save(doc); });
}

where is JUUID defined?