Javascript and 'undefined'

I like this course on JavaScript. Yesterday, I remembered what I learned about 'undefined'. He recommends not ever setting a variable to 'undefined'. When I searched the project I am working on, I saw many, many cases where it was being set in the jquery and modernizr libraries.

Either way, is it a valuable distinction to know the difference between "is not defined", null and undefined.

If you have an account you can find this lesson after you have logged in by going here.

You can find it in the course overview in the "Execution Contexts and Lexical Environments". The lesson is called "Conceptual Aside: Javascript and 'undefined'"