diff --git a/demo.html b/demo.html
index 320444a..7e8cb88 100644
--- a/demo.html
+++ b/demo.html
@@ -3,10 +3,10 @@
 <head>
     <meta charset="UTF-8">
     <meta name="viewport" content="width=device-width, initial-scale=1.0">
-    <title>FilterRange Function Demo</title>
+    <title>FilterRange and SortDescending Function Demo</title>
 </head>
 <body>
-    <h1>FilterRange Function Demo</h1>
+    <h1>FilterRange and SortDescending Function Demo</h1>
     <p>Open the browser console to see the results.</p>
     
     <script>
@@ -31,6 +31,25 @@ <h1>FilterRange Function Demo</h1>
         // Log to console as well
         console.log('Original array:', arr);
         console.log('Filtered array:', filtered);
+
+        /**
+         * Sorts an array in descending order (내림차순)
+         * @param {number[]} arr - The input array
+         * @returns {number[]} A new array sorted in descending order
+         */
+        function sortDescending(arr) {
+            return [...arr].sort((a, b) => b - a);
+        }
+
+        // Example from the problem statement
+        let arr2 = [5, 2, 1, -10, 8];
+        let sorted = sortDescending(arr2);
+
+        alert(`Original: ${arr2}, Sorted (descending): ${sorted}`); // Original: 5,2,1,-10,8, Sorted (descending): 8,5,2,1,-10
+
+        // Log to console as well
+        console.log('Original array for sorting:', arr2);
+        console.log('Sorted array (descending):', sorted);
     </script>
 </body>
 </html>
\ No newline at end of file
diff --git a/sortDescending.js b/sortDescending.js
new file mode 100644
index 0000000..77d1833
--- /dev/null
+++ b/sortDescending.js
@@ -0,0 +1,23 @@
+/**
+ * Sorts an array in descending order (내림차순)
+ * @param {number[]} arr - The input array
+ * @returns {number[]} A new array sorted in descending order
+ */
+function sortDescending(arr) {
+    return [...arr].sort((a, b) => b - a);
+}
+
+// Export for use in other modules (if using Node.js)
+if (typeof module !== 'undefined' && module.exports) {
+    module.exports = sortDescending;
+}
+
+// Example usage as shown in the problem statement
+if (typeof window === 'undefined') {
+    // Running in Node.js environment
+    let arr = [5, 2, 1, -10, 8];
+    let sorted = sortDescending(arr);
+    
+    console.log('Original array:', arr); // Should show [5, 2, 1, -10, 8] (not modified)
+    console.log('Sorted array (descending):', sorted); // Should show [8, 5, 2, 1, -10]
+}
\ No newline at end of file
diff --git a/testSort.js b/testSort.js
new file mode 100644
index 0000000..e46305b
--- /dev/null
+++ b/testSort.js
@@ -0,0 +1,42 @@
+// Test file for sortDescending function
+const sortDescending = require('./sortDescending.js');
+
+function test(description, actual, expected) {
+    const actualStr = JSON.stringify(actual);
+    const expectedStr = JSON.stringify(expected);
+    const passed = actualStr === expectedStr;
+    
+    console.log(`${passed ? '✅' : '❌'} ${description}`);
+    if (!passed) {
+        console.log(`   Expected: ${expectedStr}`);
+        console.log(`   Actual:   ${actualStr}`);
+    }
+    return passed;
+}
+
+function runSortTests() {
+    console.log('Running sortDescending tests...\n');
+    
+    let allPassed = true;
+    
+    // Test case from the problem statement
+    let arr1 = [5, 2, 1, -10, 8];
+    let sorted1 = sortDescending(arr1);
+    allPassed &= test('Problem statement test case [5, 2, 1, -10, 8]', sorted1, [8, 5, 2, 1, -10]);
+    allPassed &= test('Original array not modified', arr1, [5, 2, 1, -10, 8]);
+    
+    // Basic test cases
+    allPassed &= test('Basic positive numbers', sortDescending([3, 1, 4, 1, 5]), [5, 4, 3, 1, 1]);
+    allPassed &= test('Mixed positive and negative', sortDescending([3, -1, 4, -5, 0]), [4, 3, 0, -1, -5]);
+    allPassed &= test('All negative numbers', sortDescending([-3, -1, -4, -2]), [-1, -2, -3, -4]);
+    
+    // Essential edge cases
+    allPassed &= test('Empty array', sortDescending([]), []);
+    allPassed &= test('Single element', sortDescending([42]), [42]);
+    allPassed &= test('All same elements', sortDescending([7, 7, 7]), [7, 7, 7]);
+    
+    console.log(`\n${allPassed ? '🎉 All sort tests passed!' : '💥 Some sort tests failed!'}`);
+    return allPassed;
+}
+
+runSortTests();
\ No newline at end of file
