es6 이전 javascript에서 객체는 순서를 보장하지 않는다.
https://stackoverflow.com/questions/5525795/does-javascript-guarantee-object-property-order/23202095
4.3.3 Object An object is a member of the type Object. It is an unordered collection of properties each of which contains a primitive value, object, or function. A function stored in a property of an object is called a method.
http://www.ecma-international.org/publications/files/ECMA-ST-ARCH/ECMA-262, 3rd edition, December 1999.pdf
그러나 es6 이후 주요 벤더들은 위와 같은 공통된 구현 방식을 채택했다.
https://stackoverflow.com/questions/5525795/does-javascript-guarantee-object-property-order/38218582#38218582
getOwnPropertyNames(), keys() 차이
https://stackoverflow.com/questions/22658488/object-getownpropertynames-vs-object-keys