JSFiddle - React, Tailwind, and code Playground

JavaScript

function O() {
  this.foo = 'foo';
}
O.prototype = { bar: 'bar' }

var o = new O();

for(var v in o) { 
    console.log(v); // foo bar
}

Object.getOwnPropertyNames(o).forEach(function(val, idx, array) {
  console.log("getOwnPropertyNames() " + val);
});

Object.keys(o).forEach(function(val, idx, array) {
  console.log("keys() " + val);
});