<div class="vfx-image-wrap special">
<div class="before-image" style="width: 50%; transition: all 0.3s ease 0s;">
<img alt="" src="js/VFX-before.jpg">
</div>
<div class="after-image">
<img alt="" src="js/VFX-after.jpg">
</div>
<div class="divider-bar" style="left: 50%; transition: all 0.3s ease 0s;"></div>
</div>

$(function () {
//外DIV
var imageWrap = $('.vfx-image-wrap'),
//前景圖
topImage = $(this).find('.before-image'),
//分割線
divider = $(this).find('.divider-bar'),
stayBounce = $('.toggle-function');

imageWrap.on("mousemove", function (e) {
// Gotta localize top image and divider so it only applies to this
var offsets = $(this).offset(),
fullWidth = $(this).width(),
mouseX = e.pageX - offsets.left,
topImage = $(this).find('.before-image'),
divider = $(this).find('.divider-bar');

if (mouseX < 0) {
mouseX = 0;
} else if (mouseX > fullWidth) {
mouseX = fullWidth
}
$(this).addClass('special');
divider.css({ left: mouseX, transition: 'none' });
topImage.css({ width: mouseX, transition: 'none' });
});
stayBounce.click(function(){
$(this).toggleClass('stay');
});

imageWrap.on("mouseleave", function () {
if (!stayBounce.hasClass('stay')) {
divider.css({ left: '50%', transition: 'all .3s' });
topImage.css({ width: '50%', transition: 'all .3s' });
}
});
});