JSFiddle - React, Tailwind, and code Playground

JavaScript

function addProperty(object, name, initialValue) {
	//Просто попытка
  var initvalue = initialValue;
  
  object['set' + addProperty.upperCaseFirstLetter(name)] = function(value) {
  	initvalue = value;
  };
  
  object['get' + addProperty.upperCaseFirstLetter(name)] = function() {
  	return initvalue;
  };
}

addProperty.upperCaseFirstLetter = function(word) {
	return word[0].toUpperCase() + word.slice(1);
}


var obj = { };
addProperty(obj, 'test', 10);
console.log(obj.getTest()); // 10
obj.setTest(100);
console.log(obj.getTest()); // 100
console.log(obj); // { getTest:..., setTest: ...} - самого свойства тут нет