Dynamically generated JavaScript files should have proper content-type header

Problem:
IE6 throws runtime errors on an error-free JavaScript file generated on the backend using JSP, PHP, ...etc.

Solution:
These runtime errors are due to IE6's inability to parse the file properly. Define the content-type of the document and set it to text/javascript. This will properly format the document and will remove any runtime errors thrown in IE6.

Check for extended properties inside for..in loops

Problem:
Running a for..in loop on an object returns all enumerated properties of that object, including any methods/properties added using the prototype property. In the age of ubiquitous JavaScript libraries such as Prototype that does extend built-in objects, not accounting for those extended properties could break your code.

Solution:
it's essential to check for those extended methods and properties whenever the for..in loop is used. This is done by using the hasOwnProperty() Object method.

for (var elem in FunObject) {
   if (FunObject.hasOwnProperty()) {
       // proceed with code
   }
}

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
  }