Wonderful article on JS Delete

http://perfectionkills.com/understanding-delete/

Here’s a short summary of how deletion works in Javascript:

• Variables and function declarations are properties of either Activation or Global objects.
• Properties have attributes, one of which — DontDelete — is responsible for whether a property can be deleted.
• Variable and function declarations in Global and Function code always create properties with DontDelete.
• Function arguments are also properties of Activation object and are created with DontDelete.
• Variable and function declarations in Eval code always create properties without DontDelete.
• New properties are always created with empty attributes (and so without DontDelete).
• Host objects are allowed to react to deletion however they want.