const deepcopy = require('deepcopy');

const src = {
  desserts: [
    { name: 'cake'      },
    { name: 'ice cream' },
    { name: 'pudding'   }
  ]
};

const dist = deepcopy(src);

src.desserts = null;

console.log(src);   // { desserts: null }
console.log(dist);  // { desserts: [ { name: 'cake' }, { name: 'ice cream' }, { name: 'pudding' } ] }

function MyClass(id) {
  this._id = id;
}

const src = {
  myClasses: [
    new MyClass(1),
    new MyClass(2),
    new MyClass(3)
  ]
};

const dest = deepcopy(base, {
  customizer(value) {
    if (target.constructor === MyClass) {
      return new MyClass(target._id);
    }
  }
});

src.myClasses = null;

console.log(src);   // { myClasses: null }
console.log(dest);  // { myClasses: [ MyClass { _id: 1 }, MyClass { _id: 2 }, MyClass { _id: 3 } ] }