I don't want to keep writing the same code over and over.

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/**
 * enable JavaScript in sketches
 */
abstract class JavaScript {
  /**
   this is effectively an header file for all your
   sketch-accessible js variables and functions.
   While JavaScript doesn't care, Processing does,
   so just add sensible typing to your variables 
   and function signatures here, even if you're
   going to ignore them on the JavaScript side.
  
   Examples:
   
     Type variable = defaultValue;
     void functor(Type arg1, Type arg2);
  **/
}

// sketch-global javascript object
JavaScript javascript;

// sketch-global bind function
void bindJavaScript(JavaScript js) { javascript = js; }

/**
  call this from Javascript using:
  
  <script type="text/javascript">
  var sketch;

  function bindJavaScriptTo(canvasId) {
    var instance = Processing.getInstanceById(canvasId);
    if(!sketch && instance && instance.bindJavaScript) {
      sketch = instance;
      sketch.bindJavaScript(this);
      return;
    }
    setTimeout(function(){bindJavaScript(canvasId);}, 250);
  }

  document.addEventListener("DOMContentLoaded", function() {
    bindJavaScript('[...id string for canvas element used...]');
  }, false);
  </script>

**/

// sketch-global flag that can tell use whether we're in Java or JavaScript.
// Since Java considers floats and ints different numbers, and JavaScript does
// not, Java will set this flag to "false", and JavaScript will set it to "true".
boolean usingJavaScriptEngine = (2.0==2);