The first way to eliminate work repetition in functions is through lazy loading. Lazy loading means that no work is done until the information is necessary.

addHandler.js

function addHandler(target, eventType, handler){
 //overwrite the existing function
 if (target.addEventListener){ //DOM2 Events
 addHandler = function(target, eventType, handler){
 target.addEventListener(eventType, handler, false);
 };
 } else { //IE
 addHandler = function(target, eventType, handler){
 target.attachEvent("on" + eventType, handler);
 };
 }
 //call the new function
 addHandler(target, eventType, handler);
}
function removeHandler(target, eventType, handler){
 //overwrite the existing function
 if (target.removeEventListener){ //DOM2 Events
 removeHandler = function(target, eventType, handler){
 target.addEventListener(eventType, handler, false);
 };
 } else { //IE
 removeHandler = function(target, eventType, handler){
 target.detachEvent("on" + eventType, handler);
 };
 }
 //call the new function
 removeHandler(target, eventType, handler);
}