gistfile1.js

function methodIsDefined(fn, obj) {
  var ns = fn.split('.');
  fn = ns.pop();
  do {
    if (!ns[0]) {
      return typeof obj[fn] === 'function';
    }
  } while(obj = obj[ns.shift()]);
  return false;
}

// Object traversal method detection:

methodIsDefined('alert', window); // true 
methodIsDefined('window.alert', window); // true
methodIsDefined('alert.apply', window); // true
methodIsDefined('alert', {}); // false 
methodIsDefined('foo', window); // false
methodIsDefined('window.foo', window); // false
methodIsDefined('foo.bar.doesNotExist', {}); // false

ops, pressed the send button too early...

Every time I need to use a method like that I wish JavaScript had support for something like lorem['ipsum.dolor.amet'] = 123 it would simplify logic in so many cases. - It would create the nested objects automatically if they doesn't exist and if you are getting the value and it doesn't exist just return undefined instead of throwing an error. Would help even more in cases where you need to pass a dynamic property name.

@millermedeiros it does! Just not the way that you want :P