Refactoring: Split shared types into seperate modules and document alot.

server/shared/utils/objects.ts

@@ -0,0 +1,19 @@
+/**
+ * Helper functions for objects.
+ */
+import { hasOwnProperty } from "../types";
+
+/**
+ * If the given value is an object this function returns the property specified by `key` if it exists.
+ *
+ * @param arg - Value to treat as an object to look up the property.
+ * @param key - Key indexing the property.
+ * @returns The property of the given object indexed by `key` or `undefined` if `arg` is not an object
+ *          or has no property `key`.
+ */
+export function getFieldIfExists(
+    arg: unknown,
+    key: PropertyKey
+): unknown | undefined {
+    return hasOwnProperty(arg, key) ? arg[key] : undefined;
+}