原型链继承：
Child()的原型作为Parent()的实例来继承Parent()的方法属性
因为所有实例都继承原型方法属性，其中一个实例对原型属性值更改后，所有实例调用该属性的值全部更改
function Parent() {}
Parent.prototype.parentSay = function() {
return 'i am parent';
}
function Child() {}
Child.prototype.childSay = function() {
return 'i am child';
}
Child.prototype = new Parent();
var par = new Parent();
var kid = new Child();
console.log(kid.parentSay()); //i am parent
构造函数继承：
在子类的构造函数内部通过call或apply来调用父类构造函数
无法实现函数的复用
function People() {
this.name = ['zhangsan','lisi','wangwu'];
}
function Person() {
People.call(this);
}
var per1 = new Person();
per1.name.push('zhanliu');
console.log(per1.name); //["zhangsan", "lisi", "wangwu", "zhanliu"]
var per2 = new Person();
console.log(per2.name); //["zhangsan", "lisi", "wangwu"]
组合继承：
将原型链继承和构造函数继承结合，最常用的继承模式
原型链继承共享的属性和方法，构造函数继承实例属性
function People(num) {
this.num = num;
this.name = ['zhangsan','lisi','wangwu'];
}
People.prototype.numCount = function() {
console.log(this.num);
}
function Person(num) {
People.call(this, num);
}
//继承方式
Person.prototype = new People();
Person.prototype.constructor = Person;
var per1 = new Person(10);
per1.name.push('zhaoliu');
console.log(per1.name); //["zhangsan", "lisi", "wangwu", "zhanliu"]
per1.numCount(); //10
var per2 = new Person(20);
console.log(per2.name); //["zhangsan", "lisi", "wangwu"]
per2.numCount(); //20
