Example of how to extend/overwrite Scriptaculous Autocompleter.Base

Override Scripty function.js

if (typeof(Autocompleter) != 'undefined') { // check if Autocompleter exists
  Autocompleter.Base.addMethods({ // adds the listed functions to Autocompleter.Base
    onKeyPress: Autocompleter.Base.prototype.onKeyPress.wrap(function(proceed, event){ // "wrap" the existing "onKeyPress" in this new function, exclude the "wrap" and just do onKeyPress: function(event) if you want to always overwrite the existing function
      var args = $A(arguments);
      args.shift(); // removes "proceed" from the arguments so only the original arguments will get passed to the original function
      // place any code here you wish to execute before the original function
      var returnVal = proceed.apply(this, args); // runs the original function
      // place any code here you wish to execute after the original function
      return returnVal;
    })
  });
}