diff --git a/Javascript/Longest_Substring_Without_Repeating_Characters.js b/Javascript/Longest_Substring_Without_Repeating_Characters.js
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+// Problem Link: https://leetcode.com/problems/longest-substring-without-repeating-characters/
+
+// Time Complexity - O(n)
+// Space Complexity - O(n)
+
+const lengthOfSubstring = (s) => {
+  //Return the length of string if it is less than or equal to 1
+  //as that would be the longest count
+  if (s.length <= 1) return s.length;
+
+  //Init values
+  //map - To keep track of chars we have seen
+  //left - To keep track of beginning of substring
+  //right - counter for our loop
+  //maxLength - maxSubstring Length
+  let map = {},
+    left = 0,
+    right = 0,
+    maxLength = 0;
+
+  //Go through loop till we get to end of string
+  while (right < s.length) {
+    //Get the value of the current character from the map
+    let currentIndex = map[s[right]];
+
+    //If it exists and is within our current substring
+    //set the left value to the character immediately after it
+    if (currentIndex !== undefined && currentIndex >= left) {
+      left = map[s[right]] + 1;
+    }
+
+    //Set value of character in map to its latest index
+    map[s[right]] = right;
+
+    //Set maxLength to the greater value between itself and the length of the current substring
+    maxLength = Math.max(maxLength, right - left + 1);
+
+    //Increase counter
+    right++;
+  }
+  return maxLength;
+};