Include code example and output explanation in README

README.md

@@ -12485,6 +12485,41 @@ If a function is called with `undefined`, the `undefined` value is treated as a
 </p>
 </details>
 
+#### 87. What is the output of below code?
+
+```javascript
+const numbers = [1, 2, 3];
+
+// Count how many numbers are odd
+let xorAccumulator = numbers.reduce((sum, value) => {
+  return sum + (value % 2);
+}, 0);
+
+// IIFE applying XOR of each element shifted by its index
+(function(arr) {
+  for (let index = 0; index < arr.length; index++) {
+    xorAccumulator ^= (arr[index] << index);
+  }
+})(numbers);
+
+console.log(xorAccumulator);
+
+```
+
+- 1: 5
+- 2: 7
+- 3: 11
+- 4: 1
+
+<details><summary><b>Answer</b></summary>
+<p>
+
+##### Answer: 3
+
+This question is really showcasing how JavaScript mixes array reduction with low-level bitwise tricks. The code first uses .reduce() to turn the array into a single value by counting how many elements are odd, then an IIFE immediately kicks in and loops through the array again, shifting each number left by its index and XOR-ing it into the accumulator. The whole vibe is about understanding how reduction works for summarizing arrays and how bit shifting plus XOR can transform values in a way that feels mathematical rather than typical JS.
+</p>
+</details>
+
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