Binding isGroup to another property

I m working on a diagram what i want is to bind the is group property to another property from node properties if possible how can i do it?
If not is there any other way to tell the diagram to make node a group except setting is group to true in node properties

Yes, the decision has to be made at the time that instance of the Node or Group is created. We cannot change the class of an existing object from Node to Group, nor vice-versa.

You will either have to remove the Part and then add it back again, or have everything be a Group all the time, and just change its appearance, perhaps by changing the category (i.e. template).

One could not use a Binding for something like this anyway, because the Part has to exist already before any Bindings are evaluated.

So i can say that for instance there is a property in my nodes that is objectType
there are different objecttypes like 1,4,6,7 etc
i cant tell the diagram to make 1 and 4 groups without parsing nodedata and explicitly adding isGroup in them?

You can customize how the GraphLinksModel decides a node data object is a group, by setting GraphLinksModel.nodeIsGroupProperty before assigning Diagram.model:

    myDiagram.model = $(go.GraphLinksModel,
        nodeIsGroupProperty: function(data) {
          var t = data.objectType;
          return t === 1 || t === 4;
            { key: 1, text: "Alpha", color: "lightblue" },
            { key: 2, text: "Beta", color: "orange" },
            { key: 3, text: "Gamma", color: "lightgreen", group: 4 },
            { key: 4, text: "Delta", color: "pink", objectType: 1 }
            { from: 1, to: 2 },
            { from: 1, to: 3 },
            { from: 2, to: 2 },
            { from: 3, to: 4 },
            { from: 4, to: 1 }

Please read GraphLinksModel | GoJS API.

i m using this to load my model so where do i define the function for object type after the model from json has loaded?

function loadModel() {
  var model = go.Model.fromJson(...);
  model.nodeIsGroupProperty = function(data) { . . . };
  myDiagram.model = model;

No need to clear the old model if you are about to replace it anyway. Unless you expect there to be an exception that you handle somehow? Probably not…