polish(theme-classic): guard against potential definition mistakes in Tabs (#5674)

packages/docusaurus-theme-common/src/utils/jsUtils.ts

@@ -0,0 +1,23 @@
+/**
+ * Copyright (c) Facebook, Inc. and its affiliates.
+ *
+ * This source code is licensed under the MIT license found in the
+ * LICENSE file in the root directory of this source tree.
+ */
+
+// A replacement of lodash in client code
+
+/**
+ * Gets the duplicate values in an array.
+ * @param arr The array.
+ * @param comparator Compares two values and returns `true` if they are equal (duplicated).
+ * @returns Value of the elements `v` that have a preceding element `u` where `comparator(u, v) === true`. Values within the returned array are not guaranteed to be unique.
+ */
+export function duplicates<T>(
+  arr: readonly T[],
+  comparator: (a: T, b: T) => boolean = (a, b) => a === b,
+): T[] {
+  return arr.filter(
+    (v, vIndex) => arr.findIndex((u) => comparator(u, v)) !== vIndex,
+  );
+}