Object.getPrototypeOf()

const prototype = {};
const object = Object.create(prototype);
console.log(Object.getPrototypeOf(object) === prototype);
// 予想される結果: true

Object.getPrototypeOf(obj)

const proto = {};
const obj = Object.create(proto);
Object.getPrototypeOf(obj) === proto; // true

Object.getPrototypeOf("foo");
// TypeError: "foo" is not an object (ES5 code)
Object.getPrototypeOf("foo");
// String.prototype                  (ES2015 code)