Undefined vs. Null

Was asked this. Presumed it's JavaScript. Answer here: http://saladwithsteve.com/2008/02/javascript-undefined-vs-null.html
Basically one must keep in mind that an uninitialized variable (var a;) is undefined NOT null.
To be accurate when comparing, i.e. if to determine whether is undefined (unset) or null (set to null) use ===, i.e. if (a===null) or if (a===undefined); don't use (a==null) because that will be true regardless of null or undefined due to type coersion.