Init-Time Branching - JavaScript Patterns

gistfile1.js

// the interface 
var utils = {
  addListener: null,
  removeListener: null 
};

// the implementation
if (typeof window.addEventListener === 'function') {
  utils.addListener = function (el, type, fn) {
    el.addEventListener(type, fn, false);
  };
  
  utils.removeListener = function (el, type, fn) {
    el.removeEventListener(type, fn, false);
  };
  
} else if (typeof document.attachEvent === 'function') { // IE 
  
  utils.addListener = function (el, type, fn) {
    el.attachEvent('on' + type, fn);
  };
  
  utils.removeListener = function (el, type, fn) {
    el.detachEvent('on' + type, fn); 
  };

} else { // older browsers
  utils.addListener = function (el, type, fn) {
    el['on' + type] = fn;
  };
  
  utils.removeListener = function (el, type, fn) {
    el['on' + type] = null;
  };
}

Not the best example (browser sniffing, etc) but a good JavaScript pattern to keep in mind.