Create Function to Copy Properties From One Object to Another

I am trying to create a function that will copy all properties from a source object and paste them to a destination object. I want to create a deep copy, so I am not using object.assign().

Here is my code:

var obj1 = {
  arr: ['a','b','c','d'], // >= 4 elements, all should be truthy
  obj: {a:1,b:2,c:3,d:4}, // >= 4 values, all should be truthy
  halfTruthyArr: [null,'b',null,'d'], // >= 4 elements, half should be falsy
  halfTruthyObj: {a:1,b:null,c:3,d:null}, // >= 4 values, half should be falsy
  string: 'This is a string.',
  reverseString: function (string) {
    if (typeof string === 'string') return string.split('').reverse().join('');
  }
};

var obj2 = {}

function extend(destination, source) {
  destination = JSON.parse(JSON.stringify(source))
}

extend(obj2,obj1)

console.log(obj2)

1. obj2 is not changed using the function. I understand the function uses pass by reference, which is why the object is not reassigned. Is there a way around this so that the object I pass in as a parameter is edited?
2. The reverseString method in obj1 does not get copied using JSON.stringify. Why is this?