Does Object.assign() create a deep copy or a shallow copy?

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