设置$('#toDate').datepicker()时，$('#fromDate')的值为空.

您应该在开始时设置默认的startDate和endDate变量，并添加 changeDate 事件监听器，用于您的日期选择器.

You should set default startDate and endDate variables at the begining and add changeDate event listener to your datepickers.

例如:

// set default dates
var start = new Date();
// set end date to max one year period:
var end = new Date(new Date().setYear(start.getFullYear()+1));

$('#fromDate').datepicker({
    startDate : start,
    endDate   : end
// update "toDate" defaults whenever "fromDate" changes
}).on('changeDate', function(){
    // set the "toDate" start to not be later than "fromDate" ends:
    $('#toDate').datepicker('setStartDate', new Date($(this).val()));
}); 

$('#toDate').datepicker({
    startDate : start,
    endDate   : end
// update "fromDate" defaults whenever "toDate" changes
}).on('changeDate', function(){
    // set the "fromDate" end to not be later than "toDate" starts:
    $('#fromDate').datepicker('setEndDate', new Date($(this).val()));
});