Description:
In JavaScript, an object has a __proto__ attribute (implicit prototype), which points to the prototype of the constructor function that constructed the object.
The function Function is special. In addition to having __proto__ attributes like other objects, it also has its own unique attributes—prototype is called the prototype object. The prototype object has a constructor attribute, which points back to the function.
Classic picture:
Next are a few questions I organized, I hope to help you!
Topic 1:
function F() {
this.a = 1;
}
var obj = new F();
console.log(obj.prototype);//打印undefined 对象只有 __proto__属性没有prototype属性，函数才有Topic 2:
Object.prototype.a = 1;
var obj = {
b: 2
};
for(var i in obj) {
console.log(i); //能迭代出原型链里面的属性，所以会打印出 b和a
}
Question 3:
Object.prototype.a = 1;
var obj = {
b: undefined
};
console.log(obj.a);//1
console.log('b' in obj);//true //能找到处于原型链里面的bar属性
console.log(obj.hasOwnProperty('a'));//false 不是自身的属性
console.log(obj.hasOwnProperty('b'));//true 是自身的属性Question 4:
function A(){
}
function B(a){
this.a = a;
}
function C(a){
if(a){
this.a = a;
}
}
A.prototype.a = 1;
B.prototype.a = 1;
C.prototype.a = 1;
//实例对象都能访问构造函数的原型对象，可以明确的是以上3个都是能拿到a的
console.log(new A().a);//1 拿到的是原型上的a=1
console.log(new B().a);//undefined this.a = a;将new B()这个实例对象的a属性这是为undefined，所以拿到的是undefined
console.log(new C(2).a);//2 将new C(2)这个实例对象的a属性这是为2，所以拿到的是2
Question 5:
function A () {
}
A.prototype.n = 1;
var b = new A();//b实例对象已经建立原型连接
//原型对象指向被改变，不会切断b实例对象的
A.prototype = {
n: 2,
m: 3
}
var c = new A();//c实例对象将根据新的原型建立连接
console.log(b.n, b.m); //1 undefined 这里拿到是改变prototype之前的
console.log(c.n, c.m); //2 3 这里拿到是改变prototype之后的Question 6:
var F = function(){};
Object.prototype.a = function(){
console.log('a')
}
Function.prototype.b = function(){
console.log('b')
}
var f = new F();
F.a(); //打印a 对象都能访问Object.prototype中的属性和方法
F.b(); //打印b F是函数，原型链 F => F.__proto__ => Function.prototype => Function.prototype.__proto__ => Object.prototype
f.a(); //打印a 对象都能访问Object.prototype中的属性和方法
f.b(); //报错f.b is not a function 因f是F实例对象，原型链 f=>f.__proto__=>F.prototype=>F.prototype.__proto__ => Object.prototype