js控制数量包含截取

<div class="usermes_index_line">
进行中的单
<div id="usermes_index_line_i2"></div>
</div>

var tnum = $('#usermes_index_line_i2').html();
var nnum = tnum.substr(0,tnum.length-1);
var tanum = Number(nnum) - 1;
$('#usermes_index_line_i2').html(tanum + '单');