diff --git a/packages/normy/src/merge-data.spec.ts b/packages/normy/src/merge-data.spec.ts
new file mode 100644
index 0000000..5d57367
--- /dev/null
+++ b/packages/normy/src/merge-data.spec.ts
@@ -0,0 +1,27 @@
+import { mergeData } from './merge-data';
+
+describe('mergeData', () => {
+  it('merges data', () => {
+    expect(mergeData({ x: 1, y: { a: 2, b: 3 } }, { x: 4 })).toEqual({
+      x: 4,
+      y: { a: 2, b: 3 },
+    });
+  });
+
+  it('deeply merges data', () => {
+    expect(mergeData({ x: 1, y: { a: 2, b: 3 } }, { y: { b: 4 } })).toEqual({
+      x: 1,
+      y: { a: 2, b: 4 },
+    });
+  });
+
+  it('does not mutate params', () => {
+    const firstParam = { x: 1, y: { a: 2, b: 3 } };
+    const secondParam = { x: 1, y: { a: 2, b: 4 }, z: 5 };
+
+    mergeData(firstParam, secondParam);
+
+    expect(firstParam).toEqual({ x: 1, y: { a: 2, b: 3 } });
+    expect(secondParam).toEqual({ x: 1, y: { a: 2, b: 4 }, z: 5 });
+  });
+});