Populate one dropdown based on selection in another

function configureDropDownLists(ddl1, ddl2) {
  var colours = ['Black', 'White', 'Blue'];
  var shapes = ['Square', 'Circle', 'Triangle'];
  var names = ['John', 'David', 'Sarah'];

  switch (ddl1.value) {
    case 'Colours':
      ddl2.options.length = 0;
      for (i = 0; i < colours.length; i++) {
        createOption(ddl2, colours[i], colours[i]);
      }
      break;
    case 'Shapes':
      ddl2.options.length = 0;
      for (i = 0; i < shapes.length; i++) {
        createOption(ddl2, shapes[i], shapes[i]);
      }
      break;
    case 'Names':
      ddl2.options.length = 0;
      for (i = 0; i < names.length; i++) {
        createOption(ddl2, names[i], names[i]);
      }
      break;
    default:
      ddl2.options.length = 0;
      break;
  }

}

function createOption(ddl, text, value) {
  var opt = document.createElement('option');
  opt.value = value;
  opt.text = text;
  ddl.options.add(opt);
}

<select id="ddl" onchange="configureDropDownLists(this,document.getElementById('ddl2'))">
  <option value=""></option>
  <option value="Colours">Colours</option>
  <option value="Shapes">Shapes</option>
  <option value="Names">Names</option>
</select>

<select id="ddl2">
</select>

(function() {

  //setup an object fully of arrays
  //alternativly it could be something like
  //{"yes":[{value:sweet, text:Sweet}.....]}
  //so you could set the label of the option tag something different than the name
  var bOptions = {
    "yes": ["sweet", "wohoo", "yay"],
    "no": ["you suck!", "common son"]
  };

  var A = document.getElementById('A');
  var B = document.getElementById('B');

  //on change is a good event for this because you are guarenteed the value is different
  A.onchange = function() {
    //clear out B
    B.length = 0;
    //get the selected value from A
    var _val = this.options[this.selectedIndex].value;
    //loop through bOption at the selected value
    for (var i in bOptions[_val]) {
      //create option tag
      var op = document.createElement('option');
      //set its value
      op.value = bOptions[_val][i];
      //set the display label
      op.text = bOptions[_val][i];
      //append it to B
      B.appendChild(op);
    }
  };
  //fire this to update B on load
  A.onchange();

})();

<select id='A' name='A'>
  <option value='yes' selected='selected'>yes
  <option value='no'> no
</select>
<select id='B' name='B'>
</select>