code: func (re *JSRE) Exec(file string) error {
  	code, err := ioutil.ReadFile(common.AbsolutePath(re.assetPath, file))
  	if err != nil {
  		return err
  	}
  	var script *otto.Script
  	re.Do(func(vm *otto.Otto) {
  		script, err = vm.Compile(file, code)
  		if err != nil {
  			return
  		}
  		_, err = vm.Run(script)
  	})
  	return err
  }
docs: // Exec(file) loads and runs the contents of a file
  // if a relative path is given, the jsre's assetPath is used