Here is an atan2 function till the one mentioned in the documentation shows up.

function atan2(var nY,var nX)
{
var nPI = 180;

if(nX > 0)//first or forth quadrent
{
return(atan(nY/nX));
}

if(nX<0)//second or third quadrent
{
if(nY>=0)//second quadrent
{
return(atan(nY/nX)+nPI);
}
else //y<0 //third quadrant
{
return(abs(atan(nY/nX))+nPI);
}

}
else //x=0;
{
if(nY>0)
{
return(nPI/2);
}

if(nY<0)
{
return(-(nPI/2));
}
else //y=0
{
//undefined
}
}

}