Question

Is that return necessary? Let’s say it’s in the middle of an action.

render(contentType:'text/json', text: ['success': true] as JSON)    
return

Answer 1

If you don’t return, any code after render will also be executed, which is often not what you want, e.g.

def someAction = {

  if (someCondition) {
    render view: 'success'
    // if we don't return execution would fall through to the code below
    return  
  }

  log.error 'something went wrong'
  render view: 'error'
}

Of course, if you use this style instead, there’s no need to return

def someAction = {

  if (someCondition) {
    render view: 'success'

  } else {    
    log.error 'something went wrong'
    render view: 'error'
  }
}

If an action only has one exit point, there’s no need to return after render

def someAction = {
    render view: 'success'
}

Just remember that the code after render will be executed if you don’t return.