如何"extend" javascript "class"并添加新内容？

Question

好吧，所以我要做的是做一个弹丸类，然后在Java中以一种方式进行基础知识类"扩展"弹丸。

我想设置父级的vars，并在子类中设置任何新的vars。

var Projectile = function(x, y){
     this.x = x;
     this.y = y;
}
Projectile.prototype.update(){
     console.log("should not be called");
}
Projectile.prototype.checkIfWallHit(){
     console.log("should be called");
}

，然后是孩子课。

var BasicShot = new function(x, y, left){
     this = new Projectile(x,y);
     this.left = left;
}
BasicShot.prototype.update(){
     console.log("should be called");
}

但是，当我尝试这样做时，当我尝试制作基础知识对象时，它说"基本次数未定义"。

那么，如何扩展弹丸类？

Answer 1

这是基本模式，请参阅注释：

// === Projectile
// The constructor
function Projectile(x, y) {
    // Do things to initialize instances...
    this.x = x;
    this.y = y;
}
// Add some methods to its prototype
Projectile.prototype.update = function() {
    // This is a method on `Projectile`
    snippet.log("Projectile#update");
};
Projectile.prototype.checkIfWallHit = function(){
    snippet.log("Projectile#checkIfWallHit");
};
// ==== BasicShot
// The constructor
function BasicShot(x, y, left) {
    // Give Projectile a chance to do its thing
    Projectile.call(this, x, y);
    this.left = left;
}
// Hook it up to `Projectile`
BasicShot.prototype = Object.create(Projectile.prototype);
BasicShot.prototype.constructor = BasicShot; // JavaScript does this by default, so let's maintain it when replacing the object
// Add methods to its prototype
BasicShot.prototype.update = function() {
    snippet.log("BasicShot#update");
};
// === Usage
snippet.log("Using Projectile");
var p = new Projectile(1, 2);
p.update(); // Projectile#update
p.checkIfWallHit(); // Projectile#checkIfWallHit
snippet.log("Using BasicShot");
var c = new BasicShot(1, 2, 3);
c.update(); // BasicShot#update
p.checkIfWallHit(); // Projectile#checkIfWallHit

<!-- Script provides the `snippet` object, see http://meta.stackexchange.com/a/242144/134069 -->
<script src="http://tjcrowder.github.io/simple-snippets-console/snippet.js"></script>

---

在ES6（下一个版本的JavaScript）时，将获得 lot 更简单：

// REQUIRES ES6!!
class Projectile {
    constructor(x, y) {
        this.x = x;
        this.y = y;
    }
    update() {
        snippet.log("Projectile#update");
    }
    checkIfWallHit() {
        snippet.log("Projectile#checkIfWallHit");
    }
};
class BasicShot extends Projectile {
    constructor(x, y, left) {
        super(x, y);
        this.left = left;
    }
    update() {
        snippet.log("BasicShot#update");
    }
}

它与上面的第一个示例做的事情相同，新的语法只是句法糖。但这真的是很好的糖。

Answer 2

获得这种继承的最佳方法是使用典型的继承。

一种一般实践是将全局函数与对象相关联并使用它来创建派生对象。（由于全球很糟糕，因此有一些方法可以规避这一点）。

Object.prototype.begetObject = function () {
    function F() {}
    F.prototype = this;
    return new F();
};
BasicShot= Projectile.begetObject();

您可以像您在问题中提出的方式相同的方式覆盖/扩展功能。

BasicShot.prototype.update = function (){
     console.log("should be called");
};

我强烈建议您浏览有关原型继承的精美文章，这绝对值得。http://javascript.crockford.com/prototypal.html

Answer 3

您可以做这个

http://jsfiddle.net/jigardafda/0qkevb0q/2/

var factory = {};
factory.getProjectileObject = function(xarg, yarg){
    var Projectile = function(x, y){
         this.x = x;
         this.y = y;
    }
    Projectile.prototype.update = function() {
         console.log("should not be called");
    }
    Projectile.prototype.checkIfWallHit = function() {
         console.log("should be called");
    }
    return new Projectile(xarg, yarg);
}
factory.getBasicshotObject = function(xarg, yarg, leftarg) {
    var BasicShot = function(left){
         this.left = left;
    }
    // Extending parents methods
    BasicShot.prototype = factory.getProjectileObject(xarg, yarg);
    BasicShot.prototype.constructor = BasicShot;
    BasicShot.prototype.update = function(){
         console.log("should be called");
    }
    return new BasicShot(leftarg);
}
i = factory.getBasicshotObject(12,34, 23);
console.log(i);