如何“扩展” javascript“类”并添加新内容？

Question

好的，所以我想做的是创建一个Projectile类，然后创建一个BasicShot类，该类以类似于Java的方式“扩展” Projectile。

我想将父级变量设置为父类，并且在子类中设置任何新变量。

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");
}

然后是孩子班。

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，当我尝试执行此操作时，当我尝试创建BasicShot对象时，它显示“未定义BasicShot”。

那么，如何扩展我的Projectile类？

Answer 1

这是基本模式，请参见注释：

 // === 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> 

---

作为ES6，JavaScript的下一个版本，即得到了很多简单的：

// 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了相同的事情，新语法只是语法糖。 但这真的是很好的糖。

Answer 2

你可以这样做

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);

Answer 3

实现此类继承的最佳方法是使用原型继承。

一种普遍的做法是将全局函数与Object关联，并使用它创建派生对象。 （由于global不好，因此也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();

您可以按照在问题中提出的相同方法，将功能覆盖/扩展到此basicShot对象。

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

我强烈建议阅读这篇关于原型继承的不错的文章，这绝对是值得的。 http://javascript.crockford.com/prototypal.html