CODEBASE: Update SaveData type to be compatible with TS 5.9 and upgrade TS (#2457)

src/utils/TypeAssertion.ts

@@ -1,4 +1,4 @@
-import type { Unknownify } from "../types";
+import type { SaveData, Unknownify } from "../types";
 
 // This function is empty because Unknownify<T> is a typesafe assertion on any object with no runtime checks needed.
 // eslint-disable-next-line @typescript-eslint/no-empty-function
@@ -89,3 +89,14 @@ export function assertNumberArray(unknownData: unknown, assertFinite = false): a
     }
   }
 }
+
+export function assertSaveData(unknownData: unknown): asserts unknownData is SaveData {
+  if (typeof unknownData !== "string" && !(unknownData instanceof Uint8Array)) {
+    console.error(unknownData);
+    throw new Error(`Invalid save data. Its type is ${getFriendlyType(unknownData)}.`);
+  }
+  if (unknownData instanceof Uint8Array && !(unknownData.buffer instanceof ArrayBuffer)) {
+    console.error(unknownData);
+    throw new Error("Invalid save data. It's Uint8Array, but its buffer is not ArrayBuffer.");
+  }
+}