JavaScript指定打印 div 层

function printdiv(printpage) {
var headstr = "<html><head><title></title></head><body>";
var footstr = "</body>";
var newstr = document.all.item(printpage).innerHTML;
var oldstr = document.body.innerHTML;
document.body.innerHTML = headstr + newstr + footstr;
window.print();
document.body.innerHTML = oldstr;
return false;
}

//网页部分 <input name="b_print" type="button" class="ipt" onClick="printdiv('div_print');" value=" 打印测试 ">

//打印的内容 <div id="div_print">