移到下個月時間裡面（要再修改寫好點裡面（要再修改寫好點））

2018/06/27;10:46am
//html -
<div id="previous-month" class="leftArrow trigger prev"><!-- onclick="preMonth()" -->
    <a href="#" class="text-primary">
        <i class="fas fa-4x fa-angle-left"></i>
    </a>
</div>
<div id="next-month" class="rightArrow trigger next"><!-- onclick="nextMonth()" -->
    <a href="#" class="text-primary">
         <i class="fas fa-4x fa-angle-right"></i>
     </a>
</div>

//js -
//每次獲取的偏移量都是 0 導致的
var leftPos = 0
function preMonth() {
    leftPos -= 1000
    if (leftPos < 0) leftPos = 0
    $('#wrapper').animate({
        scrollLeft: leftPos
    }, 300);
    console.log(leftPos);
}
function nextMonth() {
    leftPos += 1000
    $('#wrapper').animate({
        scrollLeft: leftPos
    }, 300)
    console.log(leftPos);
}