diff --git a/17_longestSequence/solution/longestSequence.js b/17_longestSequence/solution/longestSequence.js
new file mode 100644
index 00000000000..abc26ca5ca3
--- /dev/null
+++ b/17_longestSequence/solution/longestSequence.js
@@ -0,0 +1,24 @@
+function longestConsecutive(nums) {
+  if (nums.length === 0) return 0;
+
+  const numSet = new Set(nums);
+  let longest = 0;
+
+  for (const num of numSet) {
+    if (!numSet.has(num - 1)) {
+      let currentNum = num;
+      let count = 1;
+
+      while (numSet.has(currentNum + 1)) {
+        currentNum++;
+        count++;
+      }
+
+      longest = Math.max(longest, count);
+    }
+  }
+
+  return longest;
+}
+
+module.exports = longestConsecutive;
diff --git a/17_longestSequence/solution/longestSequence.spec.js b/17_longestSequence/solution/longestSequence.spec.js
new file mode 100644
index 00000000000..4177ab2d603
--- /dev/null
+++ b/17_longestSequence/solution/longestSequence.spec.js
@@ -0,0 +1,73 @@
+const longestConsecutive = require('./longestSequence');
+
+// Helper: returns the actual longest consecutive sequence
+function getLongestSequence(nums) {
+  const numSet = new Set(nums);
+  let longest = 0;
+  let bestSequence = [];
+
+  for (const num of numSet) {
+    if (!numSet.has(num - 1)) {
+      let currentNum = num;
+      let currentSeq = [currentNum];
+
+      while (numSet.has(currentNum + 1)) {
+        currentNum++;
+        currentSeq.push(currentNum);
+      }
+
+      if (currentSeq.length > longest) {
+        longest = currentSeq.length;
+        bestSequence = currentSeq;
+      }
+    }
+  }
+
+  return bestSequence;
+}
+
+// ✅ Tests with printed input, sequence, and output length
+test('should return 4 for [100, 4, 200, 1, 3, 2]', () => {
+  const input = [100, 4, 200, 1, 3, 2];
+  const result = longestConsecutive(input);
+  const sequence = getLongestSequence(input);
+  console.log('Input:', input);
+  console.log('Longest Sequence:', sequence, '| Length:', result);
+  expect(result).toBe(4);
+});
+
+test('should return 8 for [9, 1, 4, 7, 3, 2, 5, 6, 0]', () => {
+  const input = [9, 1, 4, 7, 3, 2, 5, 6, 0];
+  const result = longestConsecutive(input);
+  const sequence = getLongestSequence(input);
+  console.log('Input:', input);
+  console.log('Longest Sequence:', sequence, '| Length:', result);
+  expect(result).toBe(8);
+});
+
+test('should return 5 for [1, 2, 3, 4, 5]', () => {
+  const input = [1, 2, 3, 4, 5];
+  const result = longestConsecutive(input);
+  const sequence = getLongestSequence(input);
+  console.log('Input:', input);
+  console.log('Longest Sequence:', sequence, '| Length:', result);
+  expect(result).toBe(5);
+});
+
+test('should return 1 for [10, 5, 12, 3, 55, 30]', () => {
+  const input = [10, 5, 12, 3, 55, 30];
+  const result = longestConsecutive(input);
+  const sequence = getLongestSequence(input);
+  console.log('Input:', input);
+  console.log('Longest Sequence:', sequence, '| Length:', result);
+  expect(result).toBe(1);
+});
+
+test('should return 0 for []', () => {
+  const input = [];
+  const result = longestConsecutive(input);
+  const sequence = getLongestSequence(input);
+  console.log('Input:', input);
+  console.log('Longest Sequence:', sequence, '| Length:', result);
+  expect(result).toBe(0);
+});