Branching is a technique that allows you to encapsulate browser differences into dynamic methods that get set at run-time. Once the initialization is complete, each browser only executes the code specific to its implementation of Javascript.

Branching singleton pattern

/* SimpleXhrFactory signleton, step2 */
var SimpleXhrFactory = (function(){
  //The three branches.
  var standard = {
    createXhrObject: function(){
      return new XMLHttpRequest();
    }
  };
  var activeXNew = {
    createXhrObject: function(){
      return new ActiveXObject('Msxml2.XMLHTTP');
    }
  };
  var activeXOld = {
    createXhrObject: function(){
      return new ActiveXObject('Microsoft.XMLHTTP');
    }
  };
  
  //To assign the branch, try each method; return whatever doesn't fail.
  var testObject;
  try{
    testObject = standard.createXhrObject();
    return standard; //Return this if no error was thrown.
  }
  catch(e){
    try{
      testObject = standard.createXhrObject();
      return standard; //Return this if no error was thrown
    }
    catch(e){
      try{
        testObject = activeXOld.createXhrObject();
        return activeXOld; //Return this if no error was thrown.
      }
      catch(e){
        thrown new Error('No XHR object found in this environment.');
      }
    }
  }
})();