Addons for Retail will in most cases work on Classic with removal of any non-Classic API.

If you don't have an error handling addon, get BugSack or Swatter or enable CVar scriptErrors.

See also Patch 1.13.2/API changes.

Globe[]

Globe can tell you which API functions/events in an addon have been removed in Classic, if the TOC is set to the Retail version and with Globe: check-classic

For example: https://www.townlong-yak.com/globe/#h:2a8385bff94e98a6bad8f5f09b45a148-reads

## Interface: 110000
## Globe: check-classic

API[]

There is a list of Classic-specific API and a complete list of Classic API. The TOC version is 11501

You can use the WOW_PROJECT_ID global (defined in BNet.lua) to check for Classic or Retail.

local isClassic = (WOW_PROJECT_ID == WOW_PROJECT_CLASSIC)

• Texture:SetTexture() can no longer set color textures in Patch 7.0.3, use Texture:SetColorTexture(r, g, b [, a]) instead.
• PlaySound() only accepts SoundKitIDs in Patch 7.0.3

old: PlaySound("igMainMenuOptionCheckBoxOn")
new: PlaySound(SOUNDKIT.IG_MAINMENU_OPTION_CHECKBOX_ON)

• PlaySoundFile() only accepts FileDataIDs for game sounds in Patch 8.2.0

old: PlaySoundFile("Sound/Spells/LevelUp.ogg")
new: PlaySoundFile(569593)

Maps[]

Maps were reworked in Patch 8.0.1 and the stateful API was changed to be stateless.

• WorldMapAreaIDs were replaced with UiMapIDs.
• It's no longer needed to call SetMapToCurrentZone() and GetCurrentMapAreaID() to get the current zone UI map, instead use C_Map.GetBestMapForUnit("player")

Combat Log[]

The combat log was reworked in Patch 2.4.0

• The event payload is returned from CombatLogGetCurrentEventInfo() since Patch 8.0.1
• See COMBAT_LOG_EVENT for details.

Frame Scripts[]

Most Vanilla addons still define their frames in XML, since CreateFrame() was only added in Patch 1.10

• Parameters like this, event, arg1, arg2, arg3 are no longer globals for the script handler. This was changed in Patch 4.0.1
  They are now passed like so OnEvent(self, event, ...) or OnEvent(self, event, someVar1, someVar2, someVar3)
• The this parameter was unique to WoW frame scripts. It can generally be replaced with self

Quick fix[]

There are multiple ways to handle frame scripts in XML. For simplicity's sake we will only describe a quick fix in blue text.

• Lua

function SomeAddon_OnLoad(self)
	self:RegisterEvent("ADDON_LOADED")
end

function SomeAddon_OnEvent(self, event, addon)
	print(self, event, addon)
end

• XML

<Frame name="SomeAddon">
	<Scripts>
		<OnLoad>
			SomeAddon_OnLoad(self)
		</OnLoad>
		<OnEvent>
			SomeAddon_OnEvent(self, event, ...)
		</OnEvent>
	</Scripts>
</Frame>

Lua only[]

It's also possible to create a frame and handle the scripts in Lua without use of XML, depending on how simple the addon is.

local function OnEvent(self, event, addon)
	print(self, event, addon)
end

local f = CreateFrame("Frame")
f:RegisterEvent("ADDON_LOADED")
f:SetScript("OnEvent", OnEvent)