nuance about javascript object

my-own-property.js

const obj = {
    a: 32,
    b: 32,
    c: "Hello",
    d: 128,
};

let total = 0;
Object.prototype.parentPro = 300;
function iterateToSum(obj) {
    for (let key in obj) {
        // if (typeof obj[key] === 'number') total += obj[key]  // 492
        if (typeof obj[key] === 'number' && obj.hasOwnProperty(key)) total += obj[key] //192

    }
    return total
}

nested-property.js

var layeredObj = { a: { b: "c" }, x: 4 }
var copiedObj = Object.assign({}, layeredObj)

// change nested property: a
layeredObj.a.b = "d"

// change level 0 properties: g, x
layeredObj.g = 5
layeredObj.x = 8

// support copiedObj to delete property x
const replacer = (k, v) => { if (k === 'x') { v = 6 } else return v };

console.log('ORIGIN: ', JSON.stringify(layeredObj, null, 2), '\n')
console.log('NEW: ', JSON.stringify(copiedObj, replacer, 2))

/*

ORIGIN:  {
  "a": {
    "b": "d"
  },
  "g": 5,
  "x": 8
}

NEW:  {
  "a": {
    "b": "d"
  },
  "g": 2
}
*/