JavaScript基于原型的继承的好例子

时间：2010-01-14 14:28:15

标签： javascript prototypal-inheritance

我用OOP语言编程已有10多年了，但我现在正在学习JavaScript，这是我第一次遇到基于原型的继承。通过学习优秀的代码，我倾向于学得最快。什么是正确使用原型继承的JavaScript应用程序（或库）的精心编写的示例？您能否（简要地）描述如何/在何处使用原型继承，所以我知道从哪里开始阅读？

11 个答案:

答案 0 :(得分：75)

如前所述，道格拉斯·克罗克福德（Douglas Crockford）的电影对其原因进行了很好的解释，并涵盖了如何进行。但是把它放在几行JavaScript中：

// Declaring our Animal object
var Animal = function () {

    this.name = 'unknown';

    this.getName = function () {
        return this.name;
    }

    return this;
};

// Declaring our Dog object
var Dog = function () {

    // A private variable here        
    var private = 42;

    // overriding the name
    this.name = "Bello";

    // Implementing ".bark()"
    this.bark = function () {
        return 'MEOW';
    }  

    return this;
};


// Dog extends animal
Dog.prototype = new Animal();

// -- Done declaring --

// Creating an instance of Dog.
var dog = new Dog();

// Proving our case
console.log(
    "Is dog an instance of Dog? ", dog instanceof Dog, "\n",
    "Is dog an instance of Animal? ", dog instanceof Animal, "\n",
    dog.bark() +"\n", // Should be: "MEOW"
    dog.getName() +"\n", // Should be: "Bello"
    dog.private +"\n" // Should be: 'undefined'
);

然而，这种方法的问题在于，每次创建对象时都会重新创建对象。另一种方法是在原型堆栈上声明对象，如下所示：

// Defining test one, prototypal
var testOne = function () {};
testOne.prototype = (function () {
    var me = {}, privateVariable = 42;
    me.someMethod = function () {
        return privateVariable;
    };

    me.publicVariable = "foo bar";
    me.anotherMethod = function () {
        return this.publicVariable;
    };

    return me;

}());


// Defining test two, function
var testTwo = ​function() {
    var me = {}, privateVariable = 42;
    me.someMethod = function () {
        return privateVariable;
    };

    me.publicVariable = "foo bar";
    me.anotherMethod = function () {
        return this.publicVariable;
    };

    return me;
};


// Proving that both techniques are functionally identical
var resultTestOne = new testOne(),
    resultTestTwo = new testTwo();

console.log(
    resultTestOne.someMethod(), // Should print 42
    resultTestOne.publicVariable // Should print "foo bar"
);

console.log(
    resultTestTwo.someMethod(), // Should print 42
    resultTestTwo.publicVariable // Should print "foo bar"
);



// Performance benchmark start
var stop, start, loopCount = 1000000;

// Running testOne
start = (new Date()).getTime(); 
for (var i = loopCount; i>0; i--) {
    new testOne();
}
stop = (new Date()).getTime();

console.log('Test one took: '+ Math.round(((stop/1000) - (start/1000))*1000) +' milliseconds');



// Running testTwo
start = (new Date()).getTime(); 
for (var i = loopCount; i>0; i--) {
    new testTwo();
}
stop = (new Date()).getTime();

console.log('Test two took: '+ Math.round(((stop/1000) - (start/1000))*1000) +' milliseconds');

内省时有一点点缺点。倾倒testOne，会导致信息量减少。此外，“testOne”中的私有财产“privateVariable”在所有情况下都是共享的，在shesek的回复中也有帮助。

答案 1 :(得分：47)

道格拉斯·克罗克福德在JavaScript Prototypal Inheritance上有一个很好的页面：

  

五年前，我在JavaScript中编写了Classical Inheritance。它表明JavaScript是一种无类别的原型语言，它具有足够的表达能力来模拟经典系统。从那时起，我的编程风格就已经发生了变化，就像任何优秀的程序员一样。我学会了完全接受原型主义，并将自己从经典模型的范围中解放出来。

Dean Edward的Base.js，Mootools's Class或John Resig's Simple Inheritance作品是JavaScript中classical inheritance的工作方式。

答案 2 :(得分：26)

function Shape(x, y) {
    this.x = x;
    this.y = y;
}

// 1. Explicitly call base (Shape) constructor from subclass (Circle) constructor passing this as the explicit receiver
function Circle(x, y, r) {
    Shape.call(this, x, y);
    this.r = r;
}

// 2. Use Object.create to construct the subclass prototype object to avoid calling the base constructor
Circle.prototype = Object.create(Shape.prototype);

答案 3 :(得分：14)

我会看看YUI和Dean Edward的Base图书馆：http://dean.edwards.name/weblog/2006/03/base/

对于YUI，您可以快速查看lang module，尤其是。 YAHOO.lang.extend方法。然后，您可以浏览一些小部件或实用程序的源代码，并了解它们如何使用该方法。

答案 4 :(得分：5)

还有Microsoft的ASP.NET Ajax库，http://www.asp.net/ajax/。

还有很多好的MSDN文章，包括 Create Advanced Web Applications With Object-Oriented Techniques 。

答案 5 :(得分：4)

ES6 class和extends

ES6 class和extends只是以前可能的原型链操作的语法糖，因此可以说是最规范的设置。

首先在https://stackoverflow.com/a/23877420/895245

了解有关原型链和.属性查找的更多信息

现在让我们解构会发生什么：

class C {
    constructor(i) {
        this.i = i
    }
    inc() {
        return this.i + 1
    }
}

class D extends C {
    constructor(i) {
        super(i)
    }
    inc2() {
        return this.i + 2
    }
}

// Inheritance syntax works as expected.
(new C(1)).inc() === 2
(new D(1)).inc() === 2
(new D(1)).inc2() === 3

// "Classes" are just function objects.
C.constructor === Function
C.__proto__ === Function.prototype
D.constructor === Function
// D is a function "indirectly" through the chain.
D.__proto__ === C
D.__proto__.__proto__ === Function.prototype

// "extends" sets up the prototype chain so that base class
// lookups will work as expected
var d = new D(1)
d.__proto__ === D.prototype
D.prototype.__proto__ === C.prototype
// This is what `d.inc` actually does.
d.__proto__.__proto__.inc === C.prototype.inc

// Class variables
// No ES6 syntax sugar apparently:
// https://stackoverflow.com/questions/22528967/es6-class-variable-alternatives
C.c = 1
C.c === 1
// Because `D.__proto__ === C`.
D.c === 1
// Nothing makes this work.
d.c === undefined

没有所有预定义对象的简化图表：

      __proto__
(C)<---------------(D)         (d)
| |                |           |
| |                |           |
| |prototype       |prototype  |__proto__
| |                |           |
| |                |           |
| |                | +---------+
| |                | |
| |                | |
| |                v v
|__proto__        (D.prototype)
| |                |
| |                |
| |                |__proto__
| |                |
| |                |
| | +--------------+
| | |
| | |
| v v
| (C.prototype)--->(inc)
|
v
Function.prototype

答案 6 :(得分：3)

这是我发现的最明显的例子，来自Mixu的Node书（http://book.mixu.net/node/ch6.html）：

  

我赞成合成而不是继承：

     

组合 - 对象的功能由聚合体组成   包含其他对象实例的不同类。   继承 - 对象的功能由它自己组成   功能以及其父类的功能。如果你必须   有继承，使用普通的旧JS

     

如果必须实现继承，至少要避免使用另一个继承   非标准实现/魔术功能。这是你怎么做的   在纯ES3中实现合理的传承传真（同样长   当你遵循从未在原型上定义属性的规则时）：

function Animal(name) {
  this.name = name;
};
Animal.prototype.move = function(meters) {
  console.log(this.name+" moved "+meters+"m.");
};

function Snake() {
  Animal.apply(this, Array.prototype.slice.call(arguments));
};
Snake.prototype = new Animal();
Snake.prototype.move = function() {
  console.log("Slithering...");
  Animal.prototype.move.call(this, 5);
};

var sam = new Snake("Sammy the Python");
sam.move();
     

这与经典继承不同 - 但确实如此   标准的，可理解的Javascript并具有该功能   人们主要寻求：可链接的构造函数和调用的能力   超类的方法。

答案 7 :(得分：2)

我建议看看PrototypeJS的Class.create：
第83行@ http://prototypejs.org/assets/2009/8/31/prototype.js

答案 8 :(得分：1)

我见过的最好的例子是Douglas Crockford的JavaScript: The Good Parts。绝对值得购买，以帮助您获得平衡的语言观点。

Douglas Crockford负责JSON格式，并作为JavaScript专家在雅虎工作。

答案 9 :(得分：0)

有一个代码段JavaScript Prototype-based Inheritance，其中包含特定于ECMAScript版本的实现。它将根据当前运行时自动选择在ES6，ES5和ES3实现之间使用哪个。

答案 10 :(得分：0)

在Javascript中添加基于原型的继承的示例。

// Animal Class
function Animal (name, energy) {
  this.name = name;
  this.energy = energy;
}

Animal.prototype.eat = function (amount) {
  console.log(this.name, "eating. Energy level: ", this.energy);
  this.energy += amount;
  console.log(this.name, "completed eating. Energy level: ", this.energy);
}

Animal.prototype.sleep = function (length) {
  console.log(this.name, "sleeping. Energy level: ", this.energy);
  this.energy -= 1;
  console.log(this.name, "completed sleeping. Energy level: ", this.energy);
}

Animal.prototype.play = function (length) {
  console.log(this.name, " playing. Energy level: ", this.energy);
  this.energy -= length;
  console.log(this.name, "completed playing. Energy level: ", this.energy);
}

// Dog Class
function Dog (name, energy, breed) {
  Animal.call(this, name, energy);
  this.breed = breed;
}

Dog.prototype = Object.create(Animal.prototype);
Dog.prototype.constructor = Dog;

Dog.prototype.bark = function () {
  console.log(this.name, "barking. Energy level: ", this.energy);
  this.energy -= 1;
  console.log(this.name, "done barking. Energy level: ", this.energy);
}

Dog.prototype.showBreed = function () {
  console.log(this.name,"'s breed is ", this.breed);
}

// Cat Class
function Cat (name, energy, male) {
  Animal.call(this, name, energy);
  this.male = male;
}

Cat.prototype = Object.create(Animal.prototype);
Cat.prototype.constructor = Cat;

Cat.prototype.meow = function () {
  console.log(this.name, "meowing. Energy level: ", this.energy);
  this.energy -= 1;
  console.log(this.name, "done meowing. Energy level: ", this.energy);
}

Cat.prototype.showGender = function () {
  if (this.male) {
    console.log(this.name, "is male.");
  } else {
    console.log(this.name, "is female.");
  }
}

// Instances
const charlie = new Dog("Charlie", 10, "Labrador");
charlie.bark();
charlie.showBreed();

const penny = new Cat("Penny", 8, false);
penny.meow();
penny.showGender();

ES6使用构造函数和超级关键字使用继承要容易得多。

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