Attempt to call global 'this' (a nil value)

Question

I can see that similar questions has been asked, however I'm not really that familiar with lua coding. I'm trying to fix an old World of Warcraft vanilla addon, to run in the Classic client.

The code is as follows:

    function FHH_OnLoad()

    this:RegisterEvent("PLAYER_ENTERING_WORLD");
    this:RegisterEvent("UPDATE_MOUSEOVER_UNIT");

    -- Register Slash Commands
    SLASH_FHH1 = "/huntershelper";
    SLASH_FHH2 = "/hh";
    SlashCmdList["FHH"] = function(msg)
        FHH_ChatCommandHandler(msg);
    end
    
    local version = GetAddOnMetadata("GFW_HuntersHelper", "Version");
    GFWUtils.Print("Fizzwidget Hunter's Helper "..version.." initialized!");
    
end

And it throws the following errors;

Message: Interface\AddOns\GFW_HuntersHelper\HuntersHelper.lua:27: attempt to index global 'this' (a nil value)
Time: Tue Jun 30 09:25:14 2020
Count: 1
Stack: Interface\AddOns\GFW_HuntersHelper\HuntersHelper.lua:27: attempt to index global 'this' (a nil value)
Interface\AddOns\GFW_HuntersHelper\HuntersHelper.lua:27: in function `FHH_OnLoad'
[string "*:OnLoad"]:1: in function <[string "*:OnLoad"]:1>

Locals: (*temporary) = nil
(*temporary) = nil
(*temporary) = nil
(*temporary) = nil
(*temporary) = nil
(*temporary) = "attempt to index global 'this' (a nil value)"

I've tried playing around with the "this" statement but I'm not really sure what to do, and thought I'd see if anyone of you bright people in here would know what's going on

Answer 1

If the addon in question is quite old, then at some point in the past (2010?) the Addon API has moved from global variables to locals.

Frames are defined in XML files, like the one you posted in the comment:

<Frame name="HuntersHelperFrame">
   <Scripts>
      <OnLoad>FHH_OnLoad();</OnLoad>
      <OnEvent>
         FHH_OnEvent(event, arg1, arg2, arg3, arg4, arg5, arg6, arg7, arg8, arg9); 
      </OnEvent>
   </Scripts>
</Frame>

The elements in <Scripts> are actually called as functions with their content as function body. They are called with some arguments. You can find out what arguments using World of Warcraft API as a reference. It's not official, but it's the closest thing to reference manual there is.

For now, you are interested in Widget handlers .

Now then, the first steps you should take are:

1. Change the XML:

    <Frame name="HuntersHelperFrame"> <Scripts> <OnLoad>FHH_OnLoad(self)</OnLoad> <OnEvent> FHH_OnEvent(self, event, ...) </OnEvent> </Scripts> </Frame>

2. Change the lua to reflect that:

    function FHH_OnLoad(self) self:RegisterEvent("PLAYER_ENTERING_WORLD") -- and so on, change all occurrences of `this` to `self` -- or simply name the first argument `this` instead of `self`: -- function FHH_OnLoad(this) end -- Change all of the functions: function FHH_OnEvent(self, event, ...) -- function body end

Depending on the size of the addon it might become a big chunk of work. Sadly, that's not the end to it; be wary as the script could rely directly on the presence of global variables and do some tricks.

I guess you could try to work around it with tricks like local this = self and similar, but that might not be applicable in all cases, and may cause some problems due to how framework may parse the XML.

One final note; the API changed a lot over the years and you will most likely run into even more problems. Good luck!