例子：<input id="startDate" name="startDate" value="2015-09-14" class="jcDate " />
//法一
var val1 = document.getElementsByName("startDate")[0].value;
//法二
var val2 = document.getElementById("startDate").value;
//法三
var startDate = $(‘#startDate‘).val();
原文：http://www.cnblogs.com/zkx4213/p/4806598.html