JavaScript：一无所知
复制对象（也复制继承的属性）
let _ = {};
_.copy = function(source) {
// copy has the same prototype as source
let copy = Object.create(Object.getPrototypeOf(source));
// also iterate over the kyes which are not enumerable
Object.getOwnPropertyNames(source).forEach(function(key) {
let desc = Object.getOwnPropertyDescriptor(source, key);
// each property in the target has the same
