console.log("-----------------------使用原型实现继承---------------------------");
function Person() {
}
Person.prototype.dance = function () {};
function NinjaTestInherit() {}
//通过将NinjaTest的原型赋值给Person实例，实现NinjaTestInherit继承Person.
NinjaTestInherit.prototype = new Person();
const ninjaTestInherit = new NinjaTestInherit();
if (ninjaTestInherit instanceof NinjaTestInherit) {
console.log("ninjaTestInherit receives functionally from the NinjaTestInherit proptype");
}
if (ninjaTestInherit instanceof Person) {
console.log("... and the Person proptype");
}
if (ninjaTestInherit instanceof Object) {
console.log("... and the Object proptype");
}
if (typeof ninjaTestInherit.dance === "function") {
console.log("... and can dance!");
}
Person.prototype.sing = function () {}
if (typeof ninjaTestInherit.sing !== "function") {
console.log("ninjaTestInherit can not sing!");
} else {
console.log("ninjaTestInherit can sing!-->" + ninjaTestInherit.sing);
}
Person.prototype.dance = null;
if (ninjaTestInherit.dance != null) {
console.log("ninjaTestInherit is not null-->!" + ninjaTestInherit.dance);
} else {
console.log("ninjaTestInherit is null!-->" + ninjaTestInherit.dance);
}
if (ninjaTestInherit.constructor === NinjaTestInherit) {
console.log("The ninjaTestInherit object was created by the NinjaTestInherit constructor");
} else {
console.log("The ninjaTestInherit object was not created by the NinjaTestInherit constructor");
}
if (ninjaTestInherit.constructor === Person) {
console.log("The ninjaTestInherit object was created by the Person constructor");
} else {
console.log("The ninjaTestInherit object was not created by the Person constructor");
}
从上图发现，通过设置Person实例对象作为NinjaTestInherit构造器的原型时，我们丢失了NinjaTestInherit与NinjaTestInherit初始原型之间的关联。这是一个问题，因为constructor属性可用于检测一个对象是否由某一个函数创建的。
if (ninjaTestInherit.constructor === NinjaTestInherit) {
console.log("The ninjaTestInherit object was created by the NinjaTestInherit constructor");
} else {
console.log("The ninjaTestInherit object was not created by the NinjaTestInherit constructor");
}
通过上述方式测试：发现无法查找到NinjaTestInherit 对象的constructor属性。回到原型上，原型上也没有constructor属性，继续在原型链上追溯，在Person对象的原型上具有指向Person本身的constructor属性。事实上我们查找ninjaTestInherit 对象的构造函数，我们得到的答案是Person。
但实际上这个答案是错误的，这可能是某些严重缺陷的来源。
参考《JavaScript忍者秘籍》