`IsItemSetCollectionPieceUnlocked`

ESO Lua API function — used by 36 of the 3,030 addons we indexed (combined 5,420,155 downloads). Signature from the official ESOUIDocumentation.txt, API version 101050.

Official signature

IsItemSetCollectionPieceUnlocked(integer pieceId)

Returns: bool isUnlocked

Real usage — from Item Set Browser

			-- Show the section only if the player has progress to show or the item is uncollected
			if (noLeads < #results or not IsItemSetCollectionPieceUnlocked(GetItemLinkItemId(itemLink))) then
				extension:AddSection(GetString(SI_ANTIQUITY_F

Short excerpt from the shipping addon Item Set Browser — its ESOUI page; the code belongs to its author. Shown because documentation without a working example is half a reference.

Most-downloaded addons using it

Addon	Total downloads
Item Set Browser	1,893,031
Loot Log	1,608,502
LibMultiAccountSets	425,740
Group Loot Notifier	363,639
Bandits Gear Manager	306,643
Set Collection Marker (Sticker Book)	135,099