Javascript-排序函数的替代方法？

Question

So I'm making my "Sprite" class, and right now it works properly when it's laid out like this (alot of this is unnecessary, but might help you understand):

function Entity(tname)
    {
        if (typeof (tname) === 'undefined') tname = "Entity";
        this.tname = tname;
    }

Entity.prototype.confirmType = function(tname)
    {
        if (this.tname === tname) return true;
        else return false;
    }
Entity.prototype.constructor = Entity;

function Sprite(tname, x, y, src)
    {
        this.parent.constructor.call(this, tname);

        this.x = x;
        this.y = y;
        this.img = new Image();
        this.img.src = src;

        this.render = function()
        {
            ctx.drawImage(this.img, this.x, this.y);
        }
    }

    Sprite.prototype = Object.create(Entity.prototype);
    Sprite.prototype.constructor = Sprite;
    Sprite.prototype.parent = Entity.prototype;

var sprite = new Sprite("Lucario", 400, 400, "img/slot.png");

var update = function()
{
    sprite.render();
}

But what I want to do is make Sprite 's render function just like Entity 's confirmType function, outside the constructor.

function Sprite(tname, x, y, src)
    {
        ...
    }

    Sprite.prototype.render = function()
    {
        ctx.drawImage(this.img, this.x, this.y);
    }

Not:

function Sprite(tname, x, y, src)
    {
        ...

        this.render = function()
        {
            ctx.drawImage(this.img, this.x, this.y);
        }
    }

Basically, I want to add functions to subclasses, not just override preexisting ones. Can someone help me?

Answer 1

If I understand your issue, it may be purely an issue of the order of your Javascript statements. You don't show the whole sequence of code, but when you do this:

 Sprite.prototype = Object.create(Entity.prototype);

That replaces the entire prototype on the Sprite object so if you had previously put any methods on the prototype, they would be wiped out by this assignment. If you then want to add more methods to the Sprite prototype, just add them after you do that (not before):

 Sprite.prototype = Object.create(Entity.prototype);
 Sprite.prototype.render = function() {
    ctx.drawImage(this.img, this.x, this.y);
 }

---

If you did them in the other order, it would not work:

 Sprite.prototype.render = function() {
    ctx.drawImage(this.img, this.x, this.y);
 }
 // replaces the entire prototype object, wiping out any methods that were on it
 Sprite.prototype = Object.create(Entity.prototype);