JavaScript: Find longest substring in a given string without repeating characters

JavaScript Function: Exercise-26 with Solution

Write a JavaScript function to find longest substring in a given string without repeating characters.

Sample Solution: -

HTML Code:

<!DOCTYPE html>
  <meta charset="utf-8">
  <title>Find longest substring in a given string without repeating characters</title>


JavaScript Code:

function longest_substring_without_repeating_characters(input) {
var chars = input.split('');
var curr_char;
var str = "";
var longest_string = "";
var hash = {};
for (var i = 0; i < chars.length; i++) {
curr_char = chars[i];
if (!hash[chars[i]]) 
str += curr_char; 
hash[chars[i]] = {index:i};
if(longest_string.length <= str.length)
longest_string = str;
var prev_dupeIndex = hash[curr_char].index;
var str_FromPrevDupe = input.substring(prev_dupeIndex + 1, i);
str = str_FromPrevDupe + curr_char;
hash = {};
for (var j = prev_dupeIndex + 1; j <= i; j++) {
hash[input.charAt(j)] = {index:j};
return longest_string.length > str.length ? longest_string : str;





Flowchart: JavaScript function: Longest substring in a given a string without repeating characters

Live Demo:

See the Pen JavaScript - Find longest substring in a given a string without repeating characters-function-ex- 26 by w3resource (@w3resource) on CodePen.

Improve this sample solution and post your code through Disqus

Previous: Write a JavaScript function that accept a list of country names as input and returns the longest country name as output
Next: Write a JavaScript function that returns the longest palindrome in a given string.

What is the difficulty level of this exercise?

Test your Programming skills with w3resource's quiz.

JavaScript: Tips of the Day

How to check whether a string contains a substring in JavaScript?

ECMAScript 6 introduced String.prototype.includes:

const string = "foo";
const substring = "oo";


includes doesn't have Internet Explorer support, though. In ECMAScript 5 or older environments, use String.prototype.indexOf, which returns -1 when a substring cannot be found:

var string = "foo";
var substring = "oo";

console.log(string.indexOf(substring) !== -1);

Ref: https://bit.ly/3fFFgZv