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继承再理解

时间:2017-08-17 14:02:57      阅读:251      评论:0      收藏:0      [点我收藏+]
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。

继承再理解

原文:http://www.cnblogs.com/leelam/p/7381011.html

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