A Simple Javascript Slider

HTML5 might well have a native slider element (as part of Webforms 2.0); jQuery and MooTools have slider components, and the StackOverflow slider page points to these and other implementations.

In the spirit of NIH, I wrote my own. Rolling your own is one way to learn to appreciate a language; afterwards you get bored and start trusting the libraries. Here's a screenshot:

This slider invokes a callback function (onchange) when necessary, with the percentage value of the slider. It's up to you to translate that into dates, numbers, colours, or whatever it is you're sliding. Use like this:

<table>
  <tr>
    <th>
      Slider
    </th>

    <td id="foo_slider_min">
      <img src='/images/slider_min.png'>
    </td>
    
    <td valign="top" style="padding-top:5px;">
      <div id="foo_slider_bar" style="width:128px;background-image:url('/images/slider-bar.png');background-repeat:repeat-x;position:relative;">
         
        <div id="foo_slider_button" style="position:absolute;top:0;left:95%;">
          <img src="/images/slider-button.png" alt="editor pixel size controller"/>
        </div>
      </div>
    </td>

    <td id="foo_slider_max">
      <img src='/images/slider_max.png'>
    </td>

  </tr>
</table>

Initialise this with

var onFooChange = function(percent) {
    // update foo so it's at percent%
  }
  var sliderControl = new Slider("foo_slider", onFooChange);

If your slider's name (the 1st argument to new Slider()) is "foo_slider", it expects the following DOM elements to exist:

• foo_slider_bar - the object whose width is considered a value of 100%
• foo_slider_button - the object whose position relative to foo_slider_bar determines the value of the component
• foo_slider_min - an object which sets the slider to 0% when clicked
• foo_slider_max - an object which sets the slider to 100% when clicked

Here's the actual Slider code, use as you please -

function Slider(name, onchange) {
  var sliderBar = $(name + '_bar');
  var sliderButton = $(name + '_button');
  var sliderMin = $(name + '_min');
  var sliderMax = $(name + '_max');
  var min = 0;
  var self = this;

  var max = function() {
    return parseInt(sliderBar.style.width) - 2;
  };

  var setButtonPosition = function(px) {
    if (px > max()) {
      px = max();
    } else if (px < min) {
      px = min;
    }
    sliderButton.style.left = "" + (px - 8) + "px";
    onchange(px / max());
  };

  sliderBar.onmousedown = function(event) {
    document.onmousedown = returnFalse;

    offsets(event, sliderBar, function(y, x) {
      setButtonPosition(x);
    });

    document.onmousemove = function(event2) {
      offsets(event2, sliderBar, function(y, x) {
        setButtonPosition(x);
      });
    }

    document.onmouseup = cancelMouse;
  };

  sliderMin.onclick = function() {
    self.resetTo(0);
  };

  sliderMax.onclick = function() {
    self.resetTo(1);
  };

  this.resetTo = function(proportion) {
    setButtonPosition(proportion * max());
  }
}