Check availibility of user-defined objects before you using them

Problem:
Oftentimes, programmers take the existence of user-defined variables, properties and objects for granted, which in turn results into unstable code that works when those objects exist and fails when they don't.

Solution:
Always check the existence of the following objects before manipulating them:

• DOM objects
  

  var elem_to_test = document.getElementById('ad-unit');
  if (elem_to_test) {
  // proceed with code
  }

• Global JavaScript variable/object
  

  if (typeof YAHOO != 'undefined') {
   // proceed with code
  }
  
  OR
  
  if (window.YAHOO) {
   // proceed with code
  }

• Object property
  

  if (window.obj && obj.property) {
     // proceed with code
  }