Checkout out how the deep clone changes does not change the reference to the object in numsRef[4]. Uncomment line 16, and see how the assignment copy assigns even references, allowing for the original objects to be mutated through the 'copies'.

Deep copies Matter... why?

let numsRef = {
  1: 1,
  2: 2,
  3: 4,
  4: { 1: 'hi' }
};

let deepObj = {
  numbers: numsRef
};

let newObjectDeepClone = JSON.parse(JSON.stringify(deepObj));
newObjectDeepClone.numbers[4] = 'original unchanged';

let newObjectAssign = Object.assign({}, deepObj);
// newObjectAssign.numbers[4] = 'original changed';

console.log('original: ', deepObj);
console.log('newObjectAssign: ', newObjectAssign);
console.log('newObjectDeepClone: ', newObjectDeepClone);