How to check if a value is empty in Lua?

Question

What is the proper way to make a conditional which checks of something is or is not empty in Lua? if x == "" and f x ~= "" does not seem to work.

Answer 1

Lua is a dynamically type-based language.
Any variable can hold one of the following types: nil, boolean, number, string, table, function, thread, or userdata.
Any variable in a table (including _G, the table where globals reside) without a value gives a value of nil when indexed. When you set a table variable to nil, it essentially "undeclares" it (removing the entry from memory entirely).
When a local variable is declared, if it is not assigned immediately it is given a value of nil. Unlike table variable, when you set a local variable to nil, it does not "undeclare" it (it just has a value of nil).

In Lua, an empty string ("") is still a "value" - it's simply a string of size zero.