Programming in Lua : 1.2

Part I. The Language Chapter 1. Getting Started

1.2 - Global Variables

Global variables do not need declarations. You simply assign a value to a global variable to create it. It is not an error to access a non-initialized variable; you just get the special value nil as the result:

    print(b)  --> nil
    b = 10
    print(b)  --> 10

Usually you do not need to delete global variables; if your variable is going to have a short life, you should use a local variable. But, if you need to delete a global variable, just assign nil to it:

    b = nil
    print(b)  --> nil

After that, it is as if the variable had never been used. In other words, a global variable is existent if (and only if) it has a non-nil value.