封装函数：
1、函数形参相当于变量，不能加引号。
2、实参要和形参一一对应。
案例：鼠标移到小图上，背景展示相应放大的图片。代码如下：
1 <!DOCTYPE html> 2 <html lang="en"> 3 <head> 4 <meta charset="UTF-8"> 5 <title>淘宝鼠标展示</title> 6 <style> 7 *{ 8 margin: 0; 9 padding: 0; 10 } 11 ul,li{ 12 list-style: none; 13 } 14 .box{ 15 width: 360px; 16 height: 70px; 17 padding-top: 360px; 18 border: 1px solid #f1f1f1; 19 margin: 100px auto; 20 background: url("images/01big.jpg") no-repeat; 21 22 } 23 .box ul{ 24 border: 1px solid #f1f1f1; 25 overflow: hidden; 26 } 27 .box li{ 28 float: left; 29 } 30 31 </style> 32 <script> 33 window.onload=function(){ 34 var box=document.getElementById('box'); 35 function fn(pic,bg){//封装函数，参数传递 36 var obj=document.getElementById(pic);//变量不能加引号 37 obj.onmouseover=function(){ 38 box.style.backgroundImage=bg; 39 } 40 } 41 fn("pic1","url(images/01big.jpg)");//实参 42 fn("pic2","url(images/02big.jpg)"); 43 fn("pic3","url(images/03big.jpg)"); 44 fn("pic4","url(images/04big.jpg)"); 45 fn("pic5","url(images/05big.jpg)"); 46 } 47 48 </script> 49 </head> 50 <body> 51 <div class="box" id="box"> 52 <ul> 53 <li id="pic1" ><img src="images/01.jpg" alt=""></li> 54 <li id="pic2"><img src="images/02.jpg" alt=""></li> 55 <li id="pic3"><img src="images/03.jpg" alt=""></li> 56 <li id="pic4"><img src="images/04.jpg" alt=""></li> 57 <li id="pic5"><img src="images/05.jpg" alt=""></li> 58 </ul> 59 </div> 60 </body> 61 </html>
运行效果：
