Editing WoW:UI best practices (section)

=== Use local variables ===

One stumbling block for Lua programmers coming from other languages is the fact that all variables are global unless specified otherwise. Many times, the use of global variables is necessary: saved variables are created via the global environment; all API functions and UI frames are in the global namespace. However, globals come at a cost of both performance, and naming conflicts.

Globals are inefficient to access compared to locals. Whenever a global is accessed, Lua has to call the internal equivalent of getfenv() to figure out where to retrieve the global from. Local variables, on the other hand, are stored on a stack. In code where performance is important (for instance an [[UIHANDLER OnUpdate|OnUpdate handler]]), it's often beneficial to create a local alias to a global function:

 local Foo = Foo
 
 for i = 1, 10000 do
     Foo()
 end

Naming conflicts are sometimes a more pressing concern. The effects are usually much more noticeable. If two addons define print functions that work in slightly different ways, one of the addons isn't going to be too happy when it tries to use its own. Another problem can be taint... If you accidentally use the name of some variable that blizzard code uses (e.g. arg1), you can introduce taint.

These problems are easily solved by declaring functions and variables local. There are other approaches as well, but this is by far the simplest.