When you know that a certain condition will not change throughout the life of the program, it makes sense to test the condition only once. Browser sniffing (or feature detection) is a typical example.
// BEFORE var utils = { addListener : function(el, type, fn) { if ( typeof window.addEventListener === 'function') { el.addEventListener(type, fn, false); } else if ( typeof document.attachEvent === 'function') {// IE el.attachEvent('on' + type, fn); } else {// older browsers el['on' + type] = fn; } }, removeListener : function(el, type, fn) { // pretty much the same... } }; // AFTER // 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; }; }
References:
JavaScript Patterns - by Stoyan Stefanov (O`Reilly)