Porting addons to Classic

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 . 
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: 
 ## Globe: check-classic
==API==
There is a list of Classic-specific API and a complete list of Classic API. The TOC version is 
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.
* () only accepts SoundKitIDs in Patch 7.0.3
 old: PlaySound("igMainMenuOptionCheckBoxOn")
 new: PlaySound(SOUNDKIT.IG_MAINMENU_OPTION_CHECKBOX_ON)
* () 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 () and () to get the current zone UI map, instead use ("player")
==Combat Log==
The combat log was reworked in Patch 2.4.0
* The event payload is returned from () since Patch 8.0.1
* See COMBAT_LOG_EVENT for details.
==Frame Scripts==
Most Vanilla addons still define their frames in XML, since () 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 4.0.1They 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)