摘要：面向對象主要知識點小結，基于構造函數可以理解為通過即將創建的對象將類實例化給一個對象賦予屬性或者方法原型便于方法的重用與構造函數模式相比，使用原型對象的好處是可以讓所有對象實例共享它所包含的屬性和方法。

JavaScript面向對象主要知識點小結，基于ECMAScript 5.

function People(name){
    //this可以理解為通過new即將創建的對象
    this.name = name;
}
//將類實例化
var person = new People("Cassie Xu");
console.log(person.name);

給一個對象賦予屬性或者方法

function People(name) {
  this.name = name;
  this.greet = function() {
    console.log("Hello, my name is " + this.name + "!");
  };
}
var person = new People("Cassie Xu");
person.greet();

why we use prototype? -> 便于方法的重用
與構造函數模式相比，使用原型對象的好處是可以讓所有對象實例共享它所包含的屬性和方法。
運行時沒找到函數的方法時，對象會首先找它的類的prototype方法
Demo1

function People(name) {
  this.name = name;
}
People.prototype = {
  greet: function() {
    console.log("Hello, my name is " + this.name + "!");
  }
};
var person = new People("Cassie Xu");
person.greet();
//在每個實例化后的對象都會有一個__proto__屬性,類會將它的prototype賦給實例
//實例化一個對象時，People這個類首先會將person的__proto屬性指向People.prototype
console.log(person.__proto__ === People.prototype); // true

Demo2

var a = { foo: 1 };
var b = { bar: 2 };
b.__proto__ = a;
//b本身沒有foo屬性，但是b會繼續尋找__proto__屬性
console.log(b.foo); // 1
console.log(b.bar); // 2

var a = {};
console.log(a.__proto__); // {}
console.log(a.__proto__.__proto__); // null

var a = {};
console.log(a.__proto__); // {}
console.log(a.__proto__.__proto__); // null

Demo1

function Parent(){}
Parent.prototype = {
    greet: function(){
    console.log("hello world");
}
}
function Child(){}
//此方法適用于父類Child不需要傳參數
Child.prototype = new Parent();
var c = new Child();
//c首先尋找自身的方法，沒有great,所以找Child的原型方法，而Child.prototype等于Parent方法
c.greet(); //console.log("hello world");

上面的例子如果Parent有參數，將存在以下問題：

function Parent(a, b) {}
Parent.prototype.greet = function() {
  console.log("JavaScript rocks");
}
function Child(a, b) {}
Child.prototype = new Parent(); //something wrong?->new Parent()不能傳參數，否則參數一直不變
var child = new Child();
child.greet();

Demo2
解決父類參數問題

function Parent() {
    this.name = "xxx",
    this.date = "xxx"
}
Parent.prototype.greet = function() {
  console.log("JavaScript rocks");
}
function Child() {}
//此方法適用于
Child.prototype = Object.create(Parent.prototype);
//硬記的知識 
Child.prototype.constructor = Child;
var child = new Child();
child.greet();

Demo3:繼承的一個實例
創建一個矩形

function Rect(width,height){
    this._setupDOM(width,height);
}
Rect.prototype._setupDOM = function(width,height){
    this._dom = document.createElement("div");
    this._dom.style.width = width + "px";
    this._dom.style.height = height + "px";
};
Rect.prototype.appendToBody = function(){
    document.body.appendChild(this._dom);
};
function BlueRect(width,height){
  BlueRect._super.apply(this,arguments);
}
BlueRect.prototype = Object.create(Rect.prototype);
BlueRect.prototype.constructor = BlueRect;
BlueRect._super = Rect;
BlueRect.prototype._setupDOM = function(width,height){
  BlueRect._super.prototype._setupDOM.apply(this,arguments);
  this._dom.style.backgroundColor = "blue";
};
var br = new BlueRect(200,300);
br.appendToBody();

Demo:

function Rect(width,height){
    //私有屬性加_
    this._dom = document.createElement("div");
    this._dom.style.width = width + "px";
    this._dom.style.height = height + "px";
    this._dom.style.backgroundColor = "red";
}
Rect.prototype.appendToBody = function(){
    document.body.appendChild(this._dom);
};
var rect = new Rect(100,100);
rect.appendToBody();

修改上面demo:

function Rect(width,height){
    this._setupDom(width,height);
}
Rect.prototype._setupDom = function(width,height){
    //私有屬性加_
    this._dom = document.createElement("div");
    this._dom.style.width = width + "px";
    this._dom.style.height = height + "px";
    this._dom.style.backgroundColor = "red";
};
Rect.prototype.appendToBody = function(){
    document.body.appendChild(this._dom);
};
var rect = new Rect(100,100);
rect.appendToBody();

var person = {
    firstName: "Penelope",
    lastName: "Barrymore",
    sayFullName: function () {
        console.log(this.firstName + " " + this.lastName); //=> "Penelope Barrymore"
        console.log(person.firstName + " " + person.lastName); //=> "Penelope Barrymore"
    }
};
person.sayFullName();

嚴格模式下的this

funtion foo(){
    "use strict";
    console.log(this) //undefined
}

function foo(a, b) {
  console.log(this);
  console.log(a + b);
}
var fooBinding = foo.bind({ name: "Cassie Xu" });
fooBinding(1, 2);

上面code將輸出

[object Object] {
  name: "Cassie Xu"
}

call/apply方法都為調用Object方法，區別是apply將所有參數放到一個數組中去

function foo(a, b) {
  console.log(this);
  console.log(a + b);
}
foo.call({name:"Cassie Xu"}, 1, 2);
foo.apply({name:"Cassie Xu"}, [1, 2]);

why we use it? ->避免泄露全局變量

(function(c){
    var a = 1;
    var b = 2;
    console.log(a+b+c);
})(3)
// c = 3

var a = {};
a.foo = function(callback) {
  // do something and call callback in whatever way
}
a.bar = function() {
  this.num = 1;
  var that = this;
  //閉包，這里that可以訪問到a.bar的作用域
  this.foo(function(newNum) {
    that.num = newNum;
  });
  console.log(this.num);
}
a.bar();

本文轉自 JavaScript面向對象知識點小結