in slick/slick.js [939:961]
Slick.prototype.fadeSlideOut = function(slideIndex) {
var _ = this;
if (_.cssTransitions === false) {
_.$slides.eq(slideIndex).animate({
opacity: 0,
zIndex: _.options.zIndex - 2
}, _.options.speed, _.options.easing);
} else {
_.applyTransition(slideIndex);
_.$slides.eq(slideIndex).css({
opacity: 0,
zIndex: _.options.zIndex - 2
});
}
};