帮忙看看这个JavaScript分页程序，为何翻到下一页就不能翻回了?

Question

var program_NUM=7;//总的超链接数目
var program_PREPAGE=4;//每页显示2个超链接
var currentpage=1;//当前页
var pagenum=(program_NUM%program_PREPAGE==0)?parseInt(program_NUM/program_PREPAGE):parseInt(program_NUM/program_PREPAGE+1);function previous()
{
Display_main(program_PREPAGE,program_NUM,currentpage-1);
}function next()
{
Display_main(program_PREPAGE,program_NUM,currentpage+1);
}function Displaypage(currentpage)
{
    //currentpage=icurrentpage;
var strDisplayPagenation="";
    if((pagenum==1)&&(currentpage==1)) strDisplayPagenation+="";

if((pagenum>1)&&(currentpage!=1)) 
strDisplayPagenation+='<br><a href="javascript:void(0)" onclick="javascript:previous()">上一页</a>&nbsp;&nbsp;'; 

else if((pagenum>1)&&(currentpage==1))
strDisplayPagenation+="<br>上一页&nbsp;&nbsp;";
if(pagenum>1){
for(var i=1;i<=pagenum;i++)
{
if(i!=currentpage)
strDisplayPagenation+='<a href="javascript:void(0)" onclick="javascript:DisplayProgramList('+i+');">'+i+'</a>&nbsp;&nbsp;';
else
strDisplayPagenation+=i+"&nbsp;&nbsp;";
}
}
//strDisplayPagenation+="共 " + pagenum + " 页&nbsp;&nbsp;";

if((pagenum>1)&&(currentpage!=pagenum))
strDisplayPagenation+='<a href="javascript:void(0)" onclick="javascript:next()">下一页</a>&nbsp;&nbsp;';
else if((pagenum>1)&&(currentpage==pagenum))
strDisplayPagenation+="下一页&nbsp;&nbsp;";
document.write(strDisplayPagenation);

}function DisplayProgramList(currentpage)
{
//currentpage=iCurrentPage;
    var endcount=program_PREPAGE;
if(currentpage*program_PREPAGE>program_NUM)
endcount=program_NUM;

for(var icount = ((currentpage-1)*program_PREPAGE+1); icount <=endcount; icount++)  {      
document.writeln("<br><a href='a.html'>"+icount+"</a>"); 
document.writeln("当前页码是"+currentpage);
}
}
function Display_main(program_PREPAGE,program_NUM,currentpage)
{ 
   if(program_NUM<=program_PREPAGE)
   {
    for(var icount = 1; icount <=program_NUM; icount++)  {      
document.writeln("<br><a href='a.html'>"+icount+"</a>"); 

}
Displaypage(currentpage);
   }
   else
   {
    DisplayProgramList(currentpage);
Displaypage(currentpage);
   }
}Display_main(program_PREPAGE,program_NUM,currentpage);

Answer 1

问题出在你使用了：document.write
这样，相当于页面重写了，本页的所有变量定义和函数定义都丢失了。

Answer 2

帮你重写了一下：
var program_NUM=17;//总的超链接数目
var program_PREPAGE=4;//每页显示2个超链接
var currentpage=1;//当前页
var searchstr = location.search;
if(searchstr && searchstr.indexOf("page=") > -1){
    // 查找当前要显示的页
    searchstr = searchstr.substring(searchstr.indexOf("page=")+5);
    searchstr = searchstr.split("&")[0];
    currentpage = parseInt(searchstr,10);
    //alert(currentpage);
}var pagenum=(program_NUM%program_PREPAGE==0)?parseInt(program_NUM/program_PREPAGE):parseInt(program_NUM/program_PREPAGE+1);function previous1()
{    
    Display_main(program_PREPAGE,program_NUM,currentpage-1);
}function next1()
{        
    Display_main(program_PREPAGE,program_NUM,currentpage+1);
}function Displaypage(currentpage)
{
    //currentpage=icurrentpage;
    var strDisplayPagenation="";
    if((pagenum==1)&&(currentpage==1)) strDisplayPagenation+="";
    
    if((pagenum>1)&&(currentpage!=1)) 
        strDisplayPagenation+='<br><a href="a.htm?page=' + (currentpage-1) + '">上一页</a>&nbsp;&nbsp;'; 
        
    else if((pagenum>1)&&(currentpage==1))
        strDisplayPagenation+="<br>上一页&nbsp;&nbsp;";
    if(pagenum>1){
        for(var i=1;i<=pagenum;i++)
        {
            if(i!=currentpage)
                strDisplayPagenation+='<a href="a.htm?page=' + i + '">'+i+'</a>&nbsp;&nbsp;';
            else
                strDisplayPagenation+=i+"&nbsp;&nbsp;";
        }
    }    
    //strDisplayPagenation+="共 " + pagenum + " 页&nbsp;&nbsp;";    
    
    if((pagenum>1)&&(currentpage!=pagenum))
        strDisplayPagenation+='<a href="a.htm?page=' + (currentpage+1) + '">下一页</a>&nbsp;&nbsp;';
    else if((pagenum>1)&&(currentpage==pagenum))
        strDisplayPagenation+="下一页&nbsp;&nbsp;";
    document.write(strDisplayPagenation);
}function DisplayProgramList(currentpage)
{    
    //currentpage=iCurrentPage;
    var endcount=program_PREPAGE;    
    if(currentpage*program_PREPAGE>program_NUM)
        endcount=program_NUM;
    
    for(var icount = ((currentpage-1)*program_PREPAGE+1); icount <=endcount; icount++)  {         
        document.writeln("<br><a href='a.html'>"+icount+"</a>"); 
        document.writeln("当前页码是"+currentpage);
    }
}
    function Display_main(program_PREPAGE,program_NUM,currentpage)
{ 
   if(program_NUM<=program_PREPAGE)
   {
           for(var icount = 1; icount <=program_NUM; icount++)  {         
        document.writeln("<br><a href='a.html'>"+icount+"</a>"); 
        
        }        
        Displaypage(currentpage);
   }
   else
   {
           DisplayProgramList(currentpage);
        Displaypage(currentpage);
   }
}Display_main(program_PREPAGE,program_NUM,currentpage);

Answer 3

你点了下一页后，在点上一页的时候，你看下源码  根本就没有previous()这个方法