JavaScript 监听键盘方向键

document.onkeydown = function(ev){
              var ev = ev || window.event;
              switch(ev.keyCode){
                  case 37:
            cc.log('左');
                  case 100:
         //cc.log('1');
                      //oH3.style.left =  oH3.offsetLeft - 10 + "px";
                  break;
                  case 38:
            cc.log('上');
                  case 104:
         //cc.log('6');
                      //oH3.style.top = oH3.offsetTop - 10 + "px";
                  break;
                  case 39:
            cc.log('右');
                  case 102:
         //cc.log('45');
                      //oH3.style.left = oH3.offsetLeft + 10 + "px";
                  break;
                  case 40:
            cc.log('下');
                  case 98:
         //cc.log('3');
                      //oH3.style.top = oH3.offsetTop + 10 + "px";
                  break;
                  default:
                      console.log("请按上下左右键");
                  break;
              }
              //console.log(oH3.offsetTop+"+"+oH3.style.top);
              console.log(ev.keyCode);
          }