How to define nodeCategoryProperty function for a Model in typescript?

Question

nodeCategoryProperty function expects to have the signature - (a: ObjectData, b?: string) => string , which, IMO, should be (a: ObjectData, b?: string) => string | void (a: ObjectData, b?: string) => string | void , as this function should not return anything if it is being used as a setter.

If the second argument is supplied, the function should modify the node data object so that it has that new category name.

https://gojs.net/latest/api/symbols/Model.html#nodeCategoryProperty

For the function I have defined -

    const categoryPropertyFunction = (partData: ObjectData, category?: string): string => 
    {
      if (category) partData.type = category;
      else return partData.type;
    };

I am getting TypeScript error:

TS2366: Function lacks ending return statement and return type does not include 'undefined'.

Answer 1

Here is an example of storing the category name as a property of the "d" property on the data object:

  const model = . . .;
  model.nodeCategoryProperty = function (obj: go.ObjectData, str?: string): string {
    if (arguments.length > 1) obj.d.category = str;
    return obj.d.category;
  };

Note the use of function instead of using an arrow function, so that one can check the number of arguments .

The same kind of implementation can be used for the other "...Property" functional properties of the Model classes.