JavaScript Inheritance Object.call() undefined

Question

I am attempting to create an object-oriented approach to my program. I read that this should create an inheritance of World from Sprite with Sprite being the parent, but Sprite.call(this, imagePath) comes up as imagePath is undefined. I would assume the other variables in the call would also be undefined. How do I appropriately call the parent variables?

   function Sprite(spriteX, spriteY, spriteW, spriteH, scale, positionX, positionY, direction)
                {
                    this.imagePath = world_sprite;
                    this.spriteX = spriteX;
                    this.spriteY = spriteY;
                    this.spriteW = spriteW;
                    this.spriteH = spriteH;
                    this.scale = scale;
                    this.positionX = positionX;
                    this.positionY = positionY;
                    this.direction = direction;
                    this.speed = 5;

                    this.noGravity = false;
                    this.direction = 0;

                    //Physics stuff
                    this.velX = 0;
                    this.velY = 0;
                    this.friction = 0.98;
                };

    function World(posX, posY, direction, xOffset, yOffset)
                {
                    Sprite.call(this, imagePath, positionX, positionY, direction);

                    this.spriteX = 0;
                    this.spriteY = 0;
                    this.spriteW = 400;
                    this.spriteH = 400;
                    this.scale = 0.4;

                    this.xOffset = xOffset;
                    this.yOffset = yOffset;
                    this.lives = 3;
                };

Inside the World() function, you have no variables or parameters named imagePath, positionX, or positionY. So of course those are undefined. — Michael Geary
– Michael Geary, Commented Apr 25, 2015 at 22:09
I assumed Sprite.call would get those variables from Sprite? — Max Atkins
– Max Atkins, Commented Apr 25, 2015 at 22:15
No, Sprite.call() is calling into the Sprite() function. At the point where you make the call, there is no knowledge of what goes on inside that function. You have to use only what you have available at that point. See my answer below for one way you could do this. — Michael Geary
– Michael Geary, Commented Apr 25, 2015 at 22:19

Michael Geary · Accepted Answer · 2015-04-25 22:17:05Z

1

This is one way you could accomplish what it looks like you're trying to do:

function Sprite(
   spriteX, spriteY, spriteW, spriteH,
   scale, positionX, positionY, direction
) {
    this.imagePath = world_sprite;
    this.spriteX = spriteX;
    this.spriteY = spriteY;
    this.spriteW = spriteW;
    this.spriteH = spriteH;
    this.scale = scale;
    this.positionX = positionX;
    this.positionY = positionY;
    this.direction = direction;
    this.speed = 5;

    this.noGravity = false;
    this.direction = 0;

    //Physics stuff
    this.velX = 0;
    this.velY = 0;
    this.friction = 0.98;
};

function World(
    posX, posY, direction, xOffset, yOffset
) {
    Sprite.call( this, 0, 0, 400, 400, 0.4, posX, posY, direction );

    this.xOffset = xOffset;
    this.yOffset = yOffset;
    this.lives = 3;
};

answered Apr 25, 2015 at 22:17

Michael Geary

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1 Comment

Max Atkins Over a year ago

So this would also inherit the imagePath?

Transcendence · Accepted Answer · 2015-04-25 22:10:05Z

0

You have Sprite.call(this, imagePath, positionX, positionY, direction); but the arguments to Sprite don't match this.

answered Apr 25, 2015 at 22:10

Transcendence

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3 Comments

Max Atkins Over a year ago

So the variables need to be in the argument for Sprite? I was hoping I didn't have to pass those for them to be inherited...

Transcendence Over a year ago

You either have to define them in World itself or pass it in to Sprite so that Sprite can define it.

Transcendence Over a year ago

And how did you pass them into Sprite?

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