AddEvent & RemoveEvent cross-browser implementation

// These are cross-browser implementations of addEvent and removeEvent that account for differences
// in how addEventListener and attachEvent are supported across W3C standards browsers, and IE < IE9 standards mode.
// (IE9 compatibility mode uses attachEvent, standards mode uses addEventListener).
// The approach here attempts to solve several problems:
// 1. IE's attachEvent obliterates 'this' and the 'this' in the callback becomes 'window'
// 2. The fix, which is to call the function as a method on the object, requires that
//    each instance of the function be tagged individually (the guid) so that multiple listeners can be applied
//    to the same object
// 3. Avoid the memory leaks associated with adding a property to the object that it did not already have

// The simpler approach is to simply use the approach here:
// http://stackoverflow.com/questions/6927637/addeventlistener-in-internet-explorer
// However it has the drawback of not resolving the meaning of 'this' that IE breaks.

// Use these more complex versions only if testing shows they are needed.

function addEvent(obj, type, fn) {
  if(obj.addEventListener) {
    obj.addEventListener(type, fn, false);
  }
  else if(obj.attachEvent) {
    if (!fn.$$guid) {
      fn.$$guid = addEvent.guid++;
    }
    // The purpose of this craziness is to retain the meaning of 'this'
    // in the called function. IE obliterates 'this' and replaces it with 'window'
    const eProp = type + fn.$$guid;
    obj['e'+eProp] = fn;
    obj[eProp] = function() { obj['e'+eProp](window.event); };
    obj.attachEvent('on'+type, obj[eProp]);
  }
  else {
    obj['on'+type] = fn;
  }
}
addEvent.guid = 1;

function removeEvent(obj, type, fn) {
  if(obj.removeEventListener) {
    obj.removeEventListener(type, fn, false);
  }
  else if(obj.detachEvent) {
    var eProp = type + fn.$$guid;
    obj.detachEvent('on'+type, obj[eProp]);
    delete obj['e'+eProp];
    delete obj[eProp];
  }
  else {
    delete obj['on'+type];
  }
}

Or slightly simpler (but potentially buggy) implementations:

function addEvent(elem, evnt, func) {
  /* istanbul ignore else: non-chrome events can't be tested via Karma */
  if(elem.addEventListener)  // W3C DOM
    elem.addEventListener(evnt,func,false);
  else if(elem.attachEvent) { // IE DOM
    elem.attachEvent('on'+evnt, func);
  }
  else {
    elem['on'+evnt] = func;
  }
}

function removeEvent(elem, evnt, func) {
  /* istanbul ignore else: non-chrome events can't be tested via Karma */
  if(elem.removeEventListener)  // W3C DOM
    elem.removeEventListener(evnt,func,false);
  else if(elem.detachEvent) { // IE DOM
    elem.detachEvent('on'+evnt, func);
  }
  else { // No much to do
    delete elem['on'+evnt];
  }
}