Actionscript3, why does this happen? - actionscript-3

Question

I'm get the delta time for each frame, and this is how i do it..
private var currFrame:Number,lastFrame:Number,delta:Number,playerX:Number = 10;
public function step(e:Event):void
{
trace(playerX);
currFrame = getTimer();
delta = (currFrame - lastFrame) / 1000;
lastFrame = currFrame;
playerX += (delta);
}
And the result is 'NaN' on every frame. But this works
private var currFrame:Number,lastFrame:Number=0,delta:Number,playerX:Number = 10;
public function step(e:Event):void
{
trace(playerX);
currFrame = getTimer();
delta = (currFrame - lastFrame) / 1000;
lastFrame = currFrame;
playerX += (delta);
}
the difference if you don't notice is "lastFrame:Number=0"
Any idea why, im moving from java to Actionscript3, so the more information the better :)
gettimer is this: import flash.utils.Timer;

Answer 1

Your code, lastFrame:Number=0, is setting the value of lastFrame to 0 when it is created. If the value of lastFrame isn't set to 0, then it's value is undefined. In AS2, undefined would get treated like 0. In AS3, undefined is treated like NaN.
Doing any calculation with NaN returns NaN. For example: 5 + NaN = NaN.So the first time you run your function you are doing this:
delta = (currFrame - NaN) / 1000; //set delta to NaN
lastFrame = currFrame; // sets lastFrame to something besides NaN
playerX += (delta); //sets playerX to NaN
Now your lastFrame isn't NaN, it seems like things should be working, however, playerX is now NaN, so the line:
playerX += (delta); // will always be NaN now, even if delta is not NaN
Will keep playerX at NaN forever. Because NaN + Anything is still NaN. So NaN += Anything is always NaN.