Question

I have a contrived example in javascript to explain what I am trying to do:

I have an object:

var Item = {
  _first = undefined,
  _second = undefined,
  whole = putTogether()
};

function putTogether() {
  if (_first && _second) 
    return _first + '_' + _second;

  return '{ Invalid Values }';
}

I am trying to be able to access Item.whole as a property. Is there a way I can do this so that putTogether is evaluated every time it is accessed, rather than initially when the object is created?

I am aware I can define an anonymous function for Item.whole, but I am specifically trying to construct it so that it can be referenced as a value rather than a function.

Answer 1

You may do this :

var Item = {
    _first : undefined,
    _second : undefined
};

Object.defineProperty(Item, "whole", {
    get : function(){
        if (this._first && this._second)  return this._first + '_' + this._second;
        return '{ Invalid Values }';
    },
});

console.log(Item.whole); // prints  { Invalid Values }
Item._first = "a";
Item._second = "b";
console.log(Item.whole); // prints a_b 

Demonstration

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MDN reference of defineProperty