继承再理解

function Super(){
    this.name = 'man';
    this.arr = [1,2,3]
}
var a = new Super();
a.name = 'woman';
a.arr.push(4);
console.log(a.name);   // woman
console.log(a.arr);  // [1,2,3,4]
var b = new Super();
console.log(b.name);  // man
console.log(b.arr);  // [1,2,3]

　在没有继承的情况下，new出来的实例深度复制了构造函数中的属性

function SuperClass(){
  this.name = "women";
  this.bra = ["a","b"];
}
function SubClass(){
  this.subname = "your sister";
}
SubClass.prototype = new SuperClass();
var sub1 = new SubClass();
sub1.name = "man";
sub1.bra.push("c");
console.log(sub1.name);//man
console.log(sub1.hasOwnProperty('name'))  //true
console.log(sub1.bra);//["a","b","c"]
var sub2 = new SubClass();
console.log(sub2.hasOwnProperty('name'))  //false
console.log(sub2.name);//woman
console.log(sub2.bra);//["a","b","c"]

  证明了仅通过原型链继承的话，子类的实例共享子类的原型对象（也就是共享父类的实例，父类实例化了一次，也就是仅仅深拷贝了一次）

function SuperClass() {
  this.name = "women";
  this.bra = ["a", "b"];
}
function SubClass() {
  this.subname = "your sister";
  //将SuperClass的作用域赋予当前构造函数，实现继承
  SuperClass.call(this);
}
 
var sub1 = new SubClass();
sub1.bra.push("c");
console.log(sub1.bra);//["a","b","c"]
var sub2 = new SubClass();
console.log(sub2.bra);//["a","b"]

function SuperClass() {
  this.name = "women";
  this.bra = ["a", "b"];
}
SuperClass.prototype.sayWhat = function(){
  console.log("hello");
}
function SubClass() {
  this.subname = "your sister";
  SuperClass.call(this);
}  
var sub1 = new SubClass();
console.log(sub1.sayWhat());//TypeError: undefined is not a function

而通过借用构造函数的方法来继承的话，每个子类的实例中都有了父类构造函数的属性（深拷贝），但要是方法在构造函数中的化，那就造成浪费内存，没有实现方法的服用，

  而且仅通过这种方式实现继承的化，父类的原型中的方法继承不到；

function SuperClass() {
  this.name = "women";
  this.bra = ["a", "b"];
}
SuperClass.prototype.sayWhat = function(){
  console.log("hello");
}
function SubClass() {
  this.subname = "your sister";
  SuperClass.call(this);       //第二次调用SuperClass
}
SubClass.prototype = new SuperClass(); //第一次调用SuperClass
var sub1 = new SubClass();
console.log(sub1.sayWhat());//hello

  因此可以通过原型链 + 借用构造函数实现：每个子类的实例中都有了父类构造函数的属性（深拷贝，然后每个子类实例跟父构造函数拥有相同的属性，因此仅查找它自身，而不会     沿着原型链去查找，从而实现了改变自身不影响别人），而要复用的，一般是方法，就放在原型对象上。

转载于:https://www.cnblogs.com/leelam/p/7381011.html