Figuring out Mongoose's populate function

gistfile1.js

Document.prototype.populate = function populate () {
  //if no args are passed, don't bother doing anything
  if (0 === arguments.length) return this;

  
  var pop = this.$__.populate || (this.$__.populate = {});
  var args = utils.args(arguments);
  var fn;

  if ('function' == typeof args[args.length-1]) {
    fn = args.pop();
  }

  // allow `doc.populate(callback)`
  if (args.length) {
    // use hash to remove duplicate paths
    var res = utils.populate.apply(null, args);
    for (var i = 0; i < res.length; ++i) {
      pop[res[i].path] = res[i];
    }
  }

  if (fn) {
    var paths = utils.object.vals(pop);
    this.$__.populate = undefined;
    this.constructor.populate(this, paths, fn);
  }

  return this;
};

https://github.com/Automattic/mongoose/blob/master/bin/mongoose.js#L2415