JSFiddle - React, Tailwind, and code Playground

JavaScript

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 }';
    },
    enumerable : true,
    configurable : true}
);

console.log(Item.whole);
Item._first = "a";
Item._second = "b";
console.log(Item.whole);