Forget about deep copy, even shallow copy isn’t safe, if the object you’re copying has a property with enumerable
attribute set to false.
MDN :
The Object.assign() method only copies enumerable and own properties
from a source object to a target object
take this example
var o = {};
Object.defineProperty(o,'x',{enumerable: false,value : 15});
var ob={};
Object.assign(ob,o);
console.log(o.x); // 15
console.log(ob.x); // undefined