Let users customize the preferred reaction palette

ts/util/replaceIndex.ts

@@ -0,0 +1,16 @@
+// Copyright 2021 Signal Messenger, LLC
+// SPDX-License-Identifier: AGPL-3.0-only
+
+export function replaceIndex<T>(
+  arr: ReadonlyArray<T>,
+  index: number,
+  newItem: T
+): Array<T> {
+  if (!(index in arr)) {
+    throw new RangeError(`replaceIndex: ${index} out of range`);
+  }
+
+  const result = [...arr];
+  result[index] = newItem;
+  return result;
+}