最后的代码是什么意思？

var ele_mibSelect = document.getElementById("minSelect"),
 ele_mibHeadImg = document.getElementById("mibHeadImg");
 
if (ele_mibSelect && ele_mibHeadImg) {
 ele_mibSelect.onchange = function() {
  ele_mibHeadImg.style.width = this.value;
 };
}