slick-track width calculation

HTML

<script src="https://rawgithub.com/kenwheeler/slick/master/slick/slick.min.js"></script>
<link rel="stylesheet" href="https://rawgit.com/kenwheeler/slick/master/slick/slick.css">
<aside class="container">
    <div class="entry"><div class="inner">1</div></div>
    <div class="entry"><div class="inner">2</div></div>
    <div class="entry"><div class="inner">3</div></div>
    <div class="entry"><div class="inner">4</div></div>
     <div class="entry"><div class="inner">5</div></div>
     <div class="entry"><div class="inner">6</div></div>
     <div class="entry"><div class="inner">7</div></div>
     <div class="entry"><div class="inner">8</div></div>
     <div class="entry"><div class="inner">9</div></div>
</aside>

CSS

.container{
    height: 300px;    
}

.slick-track{
    background: black;
}

.inner { 
    margin: 0 15px 0 0; 
    background: coral;    
    height: 300px;
    transition: margin 0.2s ease;
    -moz-transition: margin 0.2s ease;
    -ms-transition: margin 0.2s ease;
    -webkit-transition: margin 0.2s ease;
}

.atEnd .inner { margin: 0 0 0 15px; }

JavaScript

var $slider = $(".container")

$slider.slick({
    slidesToShow: 3,
    slidesToScroll: 3,
    infinite: false,
    variableWidth: false
})
.on("beforeChange", function(e, slick, current, next) {
    if( next >= (slick.slideCount - slick.options.slidesToShow)) {
        $slider.addClass("atEnd");
    } else {
        $slider.removeClass("atEnd");   
    }
});