javascript - JavaScript 基于原型的继承的好例子

Question

我已经使用 OOP 语言编程超过 10 年，但我现在正在学习 JavaScript，这是我第一次遇到基于原型的继承。我倾向于通过学习好的代码来最快地学习。正确使用原型继承的 JavaScript 应用程序（或库）的编写良好的示例是什么？您能否（简要地）描述一下如何/在哪里使用原型继承，所以我知道从哪里开始阅读？

score 75 · Accepted Answer

如前所述，道格拉斯·克罗克福德的电影很好地解释了原因并涵盖了如何进行。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，它会在您每次创建对象时重新创建对象。另一种方法是在原型堆栈上声明您的对象，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');

在内省方面有一个轻微的缺点。转储 testOne，将导致有用信息减少。此外，“testOne”中的私有属性“privateVariable”在所有情况下都是共享的，shesek 的回复中也提到了这一点。

score 48 · Accepted Answer

Douglas Crockford 在JavaScript Prototypal Inheritance上有一个不错的页面：

五年前，我用 JavaScript 编写了经典继承。它表明 JavaScript 是一种无类的原型语言，并且它具有足够的表达能力来模拟经典系统。从那时起，我的编程风格就发生了变化，任何优秀的程序员都应该这样做。我学会了完全接受原型主义，并将自己从经典模型的限制中解放出来。

Dean Edward 的Base.js、Mootools 的 Class或John Resig 的 Simple Inheritance作品都是在 JavaScript中进行经典继承的方法。

score 26 · Accepted Answer

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);

score 14 · Accepted Answer

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 模块，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来源，并查看它们如何使用该方法。

score 5 · Accepted Answer

还有微软的ASP.NET Ajax 库，http://www.asp.net/ajax/。

周围也有很多很好的 MSDN 文章，包括Create Advanced Web Applications With Object-Oriented Techniques。

score 5 · Accepted Answer

这是我从 Mixu 的 Node 书 ( http://book.mixu.net/node/ch6.html ) 中找到的最清晰的示例：

我更喜欢组合而不是继承：

组合——一个对象的功能是由不同类的集合组成的，其中包含其他对象的实例。继承 - 对象的功能由它自己的功能加上其父类的功能组成。如果必须继承，请使用普通的旧 JS

如果必须实现继承，至少要避免使用另一个非标准实现/魔术函数。以下是如何在纯 ES3 中实现合理的继承复制（只要您遵循从不在原型上定义属性的规则）：
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——但它是标准的、可理解的 Javascript，并且具有人们最需要的功能：可链接的构造函数和调用超类方法的能力。

score 5 · Accepted Answer

ES6class和extends

ES6class并且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。

首先了解更多关于原型链和.属性查找的信息：https ://stackoverflow.com/a/23877420/895245

现在让我们解构发生了什么：

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：

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

score 1 · Accepted Answer

我建议查看 PrototypeJS 的 Class.create:
Line 83 @ http://prototypejs.org/assets/2009/8/31/prototype.js

score 0 · Accepted Answer

我见过的最好的例子是在 Douglas Crockford 的JavaScript: The Good Parts中。绝对值得购买，以帮助您对语言有一个平衡的看法。

Douglas Crockford负责 JSON 格式，并在 Yahoo 担任 JavaScript 大师。

score 0 · Accepted Answer

有一个片段JavaScript Prototype-based Inheritance with ECMAScript version specific implementations。它将根据当前运行时自动选择在 ES6、ES5 和 ES3 实现之间使用哪一个。

score 0 · Accepted Answer

在 Javascript 中添加基于原型的继承示例。

// 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 super 关键字更容易实现继承。

javascript - JavaScript 基于原型的继承的好例子

11 回答 11

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