refactor: iterate over previous data in mapped

pkgs/charm/src/mapped.luau

@@ -32,14 +32,18 @@ local function mapped<KI, VI, KO, VO>(selector: () -> { [KI]: VI }, mapper: (ite
 		local previousData = currentData
 		currentData = selector()
 
-		for key, cachedKey in keyCache do
+		for key in previousData do
 			if currentData[key] ~= nil then
 				continue
 			end
 
-			currentOutput = ensureCanMutateOutput(currentOutput, previousOutput)
-			currentOutput[cachedKey] = nil
-			keyCache[key] = nil
+			local cachedKey = keyCache[key]
+
+			if cachedKey then
+				currentOutput = ensureCanMutateOutput(currentOutput, previousOutput)
+				currentOutput[cachedKey] = nil
+				keyCache[key] = nil
+			end
 		end
 
 		for key, value in currentData do

pkgs/charm/tests/mapped.spec.luau

@@ -52,6 +52,7 @@ test("should not update if no values change", function()
 
 	src({ 4, 5, 6, 7 })
 	src({ 1, 2, 3 })
+	map()
 	assert(calls == 3, "should not map unchanged values")
 	assert(table.concat(map()) == "234", table.concat(map()))
 	assert(prev == map(), "should not create new table")