feat: Add Deep-Merge Nested Objects Coding Exercise

coding-exercise/deep-merge-nested-objects.md

@@ -0,0 +1,48 @@
+# Exercise: Deep-Merge Two Nested Objects With Circular References
+
+### Problem Statement
+
+Write a function **`deepMerge(objA, objB)`** that merges two JavaScript objects **without mutating** either of the original objects.
+
+The function must follow these rules:
+
+1. If a property exists only in one object, return that value.
+2. If both objects contain the same key:
+   - If both values are **plain objects**, recursively merge them.
+   - If both values are **arrays**, concatenate them.
+   - Otherwise, the value from **objB** overwrites the value from **objA**.
+3. Handle **circular references** safely using a `WeakMap` to track already merged objects.
+4. The merge must be **immutable** — return a **new** object.
+
+---
+
+### Example
+
+```js
+const firstObject = {
+  x: { y: 1 },
+  arr: [1, 2],
+};
+firstObject.self = firstObject; // Circular reference
+
+const secondObject = {
+  x: { z: 2 },
+  arr: [3],
+  extra: true,
+};
+secondObject.loop = secondObject; // Circular reference
+
+const result = deepMerge(firstObject, secondObject);
+
+/*
+Expected output:
+
+{
+  x: { y: 1, z: 2 },
+  arr: [1, 2, 3],
+  extra: true,
+  self: <circular ref>,
+  loop: <circular ref>
+}
+*/
+