added js solution to _3

Author: sambabib

Diff for: .gitignore

@@ -7,4 +7,5 @@ out/
 *.vscode/
 src/main/java/com/fishercoder/solutions/_99999RandomQuestions.java
 src/main/java/com/fishercoder/solutions/_Contest.java
-.project
+.project
+bin

Diff for: javascript/_3.js

@@ -0,0 +1,23 @@
+function lengthOfLongestSubstring(s) {
+  // Using the "sliding window" data structure.
+  // Create a javascript set to store unique characters.
+  let charSet = new Set();
+  let left = 0; // Left pointer of the sliding window.
+  let maxLength = 0;
+
+  // This moves the right pointer of the sliding window.
+  for (let right = 0; right < s.length; right++) {
+      // If the character at the right pointer is already in the set, move the left pointer.
+      while (charSet.has(s[right])) {
+          charSet.delete(s[left]);
+          left++;
+      }
+      // Add the current character at the right pointer to the set.
+      charSet.add(s[right]);
+
+      // Update the maximum length of substring without repeating characters.
+      maxLength = Math.max(maxLength, right - left + 1);
+  }
+
+  return maxLength;
+}