Let users customize the preferred reaction palette

ts/test-both/util/replaceIndex_test.ts

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+// Copyright 2021 Signal Messenger, LLC
+// SPDX-License-Identifier: AGPL-3.0-only
+
+import { assert } from 'chai';
+
+import { replaceIndex } from '../../util/replaceIndex';
+
+describe('replaceIndex', () => {
+  it('returns a new array with an index replaced', () => {
+    const original = ['a', 'b', 'c', 'd'];
+    const replaced = replaceIndex(original, 2, 'X');
+
+    assert.deepStrictEqual(replaced, ['a', 'b', 'X', 'd']);
+  });
+
+  it("doesn't modify the original array", () => {
+    const original = ['a', 'b', 'c', 'd'];
+    replaceIndex(original, 2, 'X');
+
+    assert.deepStrictEqual(original, ['a', 'b', 'c', 'd']);
+  });
+
+  it('throws if the index is out of range', () => {
+    const original = ['a', 'b', 'c'];
+
+    [-1, 1.2, 4, Infinity, NaN].forEach(index => {
+      assert.throws(() => {
+        replaceIndex(original, index, 'X');
+      });
+    });
+  });
+});