Beware of evil newbie trap when moving sprites!

[wombat]

Hi,

 

I just spent a couple of hours figuring this out:

var t;function moveSpriteUp (dt){  t + = dt;  mySprite.y = 100 + t  * 10;}

That made the sprite rotate, not move. Why? Because t isn't initialized, t + dt is NaN, and setting sprite.x or sprite.y to NaN appearently messes with references so that some other sprite's rotation affects this sprite. I guess all kinds of strange behaivious could occur.

 

This is how it shoud be:

var t = 0; 

Piously using JSHint didn't help at all...

[jerome]

I didn't know about this bug (if it is so).

It's a good practice to initialize your variables if you already know their datatype as it helps the js compiler to optimize the generated bytecode.