请问这问题,用javascript怎么写好?

Question

给一个简单好懂的
function DrawImage(ImgD,iwidth,iheight)
{
    //参数(图片,允许的宽度,允许的高度)
    var image=new Image();
    image.src=ImgD.src;    
    if(image.width>0 && image.height>0)
    {        
        if(image.width/image.height>= iwidth/iheight)
        {
            if(image.width>iwidth)
            {  
                ImgD.width=iwidth;
            ImgD.height=(image.height*iwidth)/image.width;
            }
            else
            {
                ImgD.width=image.width;  
                ImgD.height=image.height;
            }
        }
        else
        {
            if(image.height>iheight)
            {
                ImgD.height=iheight;
                ImgD.width=(image.width*iheight)/image.height;        
            }
            else
            {
                ImgD.width=image.width;  
                ImgD.height=image.height;
            }
        }
    }    
}调用方法：<img src="..." onload=DrawImage(this,450,300) />