Calculated fields in json API?

I'm wondering to which extend should we add in an API values that can be calculated from the raw data and extra available information (from the browser session, user interface.. or whatever)

For example, we could have an API returning this JSON:

{
    ownder_id: '123',
    canAddComment: true,
    etc...
}

That gives us the value "canAddComment" directly. Or we could have just this:

{
    ownder_id: '123',
    etc...
}

Where, comments can be calculated from the owner_id. For example, a user can add comments if the session owner_id is different from the received from the API.

We could have done this in Javascript instead:

//supposing we have a session object with our browser session values
var params = {owner_id: session.owner_id};
$.post(apiURL, params, function(data){
    var result = JSON.parse(data);

    //processing the data
    result["canAddComments"] = result.owner_id !== session.owner_id;

    //do whatever with it
});

What would be the best approach? What's the recommendation in this kind of cases?

Solution

I'm not exactly sure what you want to know. But my first reaction is you make a function out of it.

// constructor
function Comment() {
  this.owner_id;
  this.canAddComment = function() {
    return session.user_id === this.owner_id;
  }
}

var session = {
  user_id: 22,
  name: 'Kevin'
};

var my_comment = new Comment();
my_comment.owner_id = 15;
if(my_comment.canAddComment()) {
  $.ajax({
    // now read the form and submit the data ...
  })
}
else {
  alert('Only the author of this message can update ...');
}

EDIT:

My question was mainly if this should better be calculated in the backend side or calculated after retrieving the API data.

Not sure if there is a generic answer. A few arguments: if you want the method to be secret, you must do it on server side. In other cases, I think it makes perfect sense to let the client PC use its processing power.

Okay, one example: sorting. Let's say there is a table full of data; clicking on the head sorts the table according to that column. Does it make sense to send all the data to the server, let it sort the table and return an array of keys? No, I think it makes more sense to let the client process this. http://tablesorter.com/docs/

Similar with Google Maps: you drag a marker to some place, then the program must calculate the closest 5 bus stops. Well, obviously you calculate all of this on client side.