Return json object in controller with placeholder keyword

I am working through a node js, express js, and angular js project and the example given for a controller is not how I have it written. I cannot seem to find any articles explaining the form.

module.exports = {
  index: function(req,res){
    //your code here
    res.json({placeholder:'index'});
  }
}

I understand the res.json is returning the json object, but I'm not familiar with placeholder:'index'.

via JSim