slider.js

// simple slider

function Slider (slider, duration, startSlide) {
  this.slider     = document.getElementById(slider);
  this.slides     = this.slider.getElementsByTagName('div');
  this.duration   = duration || 1000;
  this.startSlide = startSlide || 0;
  this.init();
}

Slider.prototype.info = function() {
  console.log(
    "[ duration: " + this.duration + "]\n", 
    "[ slider count: " + this.slides.length + "]\n", 
    "[ start slide: " + this.startSlide + "]\n"
    );
};

Slider.prototype.init = function() {
  this.showSlide();
};

Slider.prototype.next = function() {
  if (this.startSlide >= this.slides.length - 1) {
    this.startSlide = 0;
  } else {
    this.startSlide++;
  }
  this.showSlide();
};

Slider.prototype.prev = function() {
  if (this.startSlide <= 0) {
    this.startSlide = this.slides.length - 1;
  } else {
    this.startSlide--;
  }
  this.showSlide();
};

Slider.prototype.showSlide = function() {
  for (var i = 0; i < this.slides.length; i++) {
    this.slides[i].className = "slide";
  }
  this.slides[this.startSlide].className += " visible";
};


Slider.prototype.autoplay = function() {
  var that = this;
  setInterval(function() {
    that.next();
  }, this.duration)
};

window.onload = function() {
  var slider = new Slider('slider', 2000);
  slider.autoplay();
}