code: function requireModel(filePath, cb) {
              //check if file exist. if not return error
              fs.exists(filePath, function (exists) {
                  if (!exists) {
                      return cb(new Error("Requested Model Not Found!"));
                  }
  
                  libProduct.model = require(filePath);
                  return cb(null, libProduct);
              });
          }
docs: checks if model file exists, requires it and returns it.
  @param filePath
  @param cb