"use strict"; class Base { } let value = 2; class Obj extends Base { set test(v) { value = v; } set() { return super.test = 3; } } const obj = new Obj(); expect(obj.set()).toBe(3); expect(Base.prototype.test).toBeUndefined(); expect(Obj.prototype.test).toBeUndefined(); // This is incorrect according to the spec, // but close enough for loose. expect(value).toBe(3); expect(obj.test).toBe(undefined);