这里的“var x_max = d_width-b_width-step; 该咋解释啊？【求大神指教】

Question

window.onload=function(){  document.body.onkeyup=function(evt){   var step=20;   var box=document.getElementById("box");   var img=document.getElementById("img");      var left = parseInt(img.style.left);   /*获得图片左边距的数值*/   var  top = parseInt(img.style.top);   /*获得图片高的数值*/      var d_width = box.offsetWidth;   var d_height = box.offsetHeight;   /*获得box控件的的绝对（宽度，高度）*/      var b_width = img.width;   var b_height = img.height;   /*获得图片的（宽度，高度）*/      var x_max = d_width-b_width-step;   var y_max = d_height-b_height-step;   /**/   var e = evt ? evt: window.event;   if(e.keyCode == 37 && left >=step){     img.style.left = left-step+"px";   }   if(e.keyCode == 38 && top >=step){     img.style.top = top-step+"px";   }   if(e.keyCode == 39 && left<= x_max){   img.style.left = left+step+"px";    }    if(e.keyCode == 40 && top <=y_max){     img.style.top = top+step+"px";   }  }}

Answer 1

这是自定义的偏移，step都没说是干什么用的