How to Check if a String is Uppercase or Lowercase in Javascript

In this tutorial, you will learn how to check if a string is uppercase or lowercase in javascript. If all the letters in a given string are in uppercase, then that string is known as an uppercase string. On the other hand, if all the letters are in lowercase, then that string is known as a lowercase string.

Most of the websites prefer certain data string to be in uppercase or lowercase and this is when it becomes very important to know how to verify if that data string is in uppercase or lowercase. This tutorial is definitely going to give you an idea about how you can accomplish that in a simple and easy way.

There are numerous ways to detect whether a string is in uppercase or lowercase. But for the sake of simplicity, we will use a strict equality operator (===), toLowerCase() and toUpperCase() methods to accomplish our goal. The toUpperCase() method converts a given string into an uppercase string and the toLowerCase() method converts a given string into a lowercase string.

In the following example, we have one global variable that holds a string. Upon click of a button, we will check whether it is in uppercase or lowercase 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 “Check” 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 attached a click event listener to the button element.
  • We have a global variable myString which holds a string as its value.
  • In the event handler function, we are using a strict equality operator (===) and the toLowerCase() method to verify whether myString is in lowercase or not. Depending upon the result of the check, we will get a Boolean value which will be stored in the isLowerCase variable.
  • Similarly, we are using the toUpperCase() method to verify whether myString is in uppercase or not and storing the Boolean value in the isUpperCase variable.
  • We are verifying the casing of the myString using the if statement and depending upon the result of the verification, we are assigning “Lower“, “Upper”, and “Mixed” 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 = "abcdef";

btnCheck.addEventListener("click", () => {
  let isLowerCase = myString.toLowerCase() === myString;
  let isUpperCase = myString.toUpperCase() === myString;

  let result;

  if (isLowerCase) {
    result = "Lower";
  } else if (isUpperCase) {
    result = "Upper";
  } else {
    result = "Mixed";

  output.innerText = result;