Creating a javascript slider; slide motion doesn't work on 1st slide, but sliding motion works after second slide?

Question

I'm building a Javascript slider for my site, and I seem to be having a strange issue. I want the sliding motion to occur when I click on my "next slide" link. When I click on "Next slide" the first time, the image simply jumps from the first to the second; however when I click "Next slide" for the second time, the image slides to the third the way I want it to. Does anyone know why the sliding motion seems to skip the initial click? See code below.

.html

 <script>
    var Slider = function() { this.initialize.apply(this, arguments) }
    Slider.prototype = {

      initialize: function(slider) {
        this.ul = slider.children[0]
        this.li = this.ul.children

        // make <ul> as large as all <li>’s
        this.ul.style.width = (this.li[0].clientWidth * this.li.length) + 'px'

        this.currentIndex = 0
      },

      goTo: function(index) {
        // filter invalid indices
        if (index < 0 || index > this.li.length - 1)
          return

        // move <ul> left
        this.ul.style.left = '-' + (100 * index) + '%'

        this.currentIndex = index
      },

      goToPrev: function() {
        this.goTo(this.currentIndex - 1)
      },

      goToNext: function() {
        this.goTo(this.currentIndex + 1)
      }
    }
  </script>

  <div class="slider">
    <ul>
      <li><img src="commercial1.png"></li>
      <li><img src="commercial2.png"></li>
      <li><img src="commercial3.png"></li>
    </ul>
  </div>

  <script>
    var sliders = []
    $('.slider').each(function() {
      sliders.push(new Slider(this))
    })
  </script>

  <div class="controls">
    <ul>
      <li><a href="javascript:sliders[0].goToPrev()">.goToPrev()</a></li>
      <li><a href="javascript:sliders[0].goToNext()">.goToNext()</a></li>
    </ul>
    </div>

.CSS

.slider {
  width: 1000px; height: 500px;
  overflow: hidden;
}
  .slider > ul {

 position: relative;
-webkit-transition: 0.5s left;
-moz-transition: 0.5s left;
-ms-transition: 0.5s left;
-o-transition: 0.5s left;

    list-style: none;
    margin: 0; padding: 0;
  }
    .slider > ul > li {
      float: left;
      width: 1000px; height: 500px;
    }

    .controls {

    padding-top:10px;   

    }

Answer 1

Reason it isn't working is because you have provided the initial state for it to transition from.

Simply adjust your css to this

  .slider > ul {
   position: relative;
   transition: left .5s ease-in-out;
   left:0;
 }

*note the left:0;

And that should do it