How to Check if a String has Unique Characters in Javascript

In this tutorial, you will learn how to check if a string has unique characters in javascript. A string is formed with a combination of multiple characters and as a result, a string can have duplicate or unique characters. For a newbie developer, it can be a bit tricky to find if a string has unique characters.

There are numerous ways to check if a string has unique characters. But for the sake of simplicity, we will use length property and Set() object to accomplish our goal. The length property returns the length of the string. If the string is empty, then it will return 0. The Set() object in javascript allows you to store type agnostic values, which means you can store any value of any data type. All the values in the collection will be unique.

In the following example, we have one global variable that holds a string. Upon click of a button, we will check if the string has  unique characters and display the result on the screen.  Please have a look over the code example and the steps given below.


  • We have 3 elements in the HTML file (div, button, and h1). The div element is just a wrapper for the rest of the elements.
  • The innerText for the button element is “Get” and for the h1 element, it is “Result”.
  • We have done some basic styling using CSS and added the link to our style.css stylesheet inside the head element.
  • We have also included our javascript file script.js with a script tag at the bottom.
<!DOCTYPE html>
<html lang="en">

  <meta charset="UTF-8">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <meta http-equiv="X-UA-Compatible" content="ie=edge">
  <link rel="stylesheet" href="style.css">


  <div class="container">    

  <script src="script.js"></script>

.container {        
    text-align: center;

button {
  margin-top: 10px;
  padding: 10px 20px;


  • We have selected the button element and h1 element using the document.querySelector() method and stored them in btnCheck and output variables respectively.
  • We have a global variable myString which holds a string as its value.
  • We have attached a click event listener to the button element.
  • In the event handler function, we are passing myString to Set constructor to get a Set() object. We are using size property to the count of the characters in the Set.
  • We are using the length property to get the count of characters in the string. If both the counts are  equal, that means the string has unique characters.
  • Depending upon the result of the check, we are assigning "Yes" or "No" to the result variable.
  • We are displaying the result in the h1 element using the innerText property.
let btnCheck = document.querySelector("button");
let output = document.querySelector("h1");

let myString = `ronald`;

btnCheck.addEventListener("click", () => {  
  let result = new Set(myString).size == myString.length ? "Yes" : "No";
  output.innerText = result;