How can I test if a letter in a string is uppercase or lowercase using JavaScript?


if (character == character.toLowerCase())
  // The character is lowercase
  // The character is uppercase

How can I test if a letter in a string is uppercase or lowercase using JavaScript?

This is straightforward, readable solution using a simple regex.

// Get specific char in string
const char = string.charAt(index);

const isLowerCaseLetter = (/[a-z]/.test(char));
const isUpperCaseLetter = (/[A-Z]/.test(char));

The problem with the other answers is, that some characters like numbers or punctuation also return true when checked for lowercase/uppercase.

I found this to work very well for it:

function isLowerCase(str)
    return str == str.toLowerCase() && str != str.toUpperCase();

This will work for punctuation, numbers and letters:


To check one letter just call it using isLowerCase(str[charIndex])

One I use (notice this doesnt make "TestString" as "T est String" or " Test String").

function seperateCapitalised(capitalisedString) {
    if (typeof capitalisedString !== "string" || capitalisedString.length === 0)
        return capitalisedString;

    var newStr = capitalisedString[0];
    for (var i = 1; i < capitalisedString.length; i++) {
        var char = capitalisedString[i];

        if (char === char.toUpperCase() && isNaN(char)) {
            newStr += ' ' + char;
        else {
            newStr += char;
    return newStr;

This is how I did it recently:

1) Check that a char/string s is lowercase

s.toLowerCase() == s && s.toUpperCase() != s

2) Check s is uppercase

s.toUpperCase() == s && s.toLowerCase() != s

Covers cases where s contains non-alphabetic chars and diacritics.

See my comment on the chosen answer. Other solutions that limit to the ASCII table or use the actual character literals completely ignore Unicode and the several hundred other characters there that have case.

This code will set the caseGroup variable to:

  • 1 for Upper Case
  • -1 for Lower Case
  • 0 for Without Case

    var caseGroup = (character.toLowerCase() == character.toUpperCase() ? 0 : (character == character.toUpperCase() ? 1 : -1));

You could bake that into something like this...

    function determineCase(character) {
        return (character.toLowerCase() == character.toUpperCase() ? 0 : (character == character.toUpperCase() ? 1 : -1));

    function isUpper(character) {
        return determineCase(character) == 1;

    function isLower(character) {
        return determineCase(character) == -1;

    function hasCase(character) {
        return determineCase(character) != 0;

More specifically to what is being asked. Pass in a String and a position to check. Very close to Josh's except that this one will compare a larger string. Would have added as a comment but I don't have that ability yet.

function isUpperCase(myString, pos) { 
    return (myString.charAt(pos) == myString.charAt(pos).toUpperCase()); 

function isLowerCase(myString, pos) {
    return (myString.charAt(pos) == myString.charAt(pos).toLowerCase()); 

This question has clearly been answered a number of times, but i thought i'd share my solution as I haven't seen it in the given answers.

var lower_case = function(letter){
    lowers = "abcdefghijklmnopqrstuvwxyz";
    return letter === letter.toLowerCase() && lowers.indexOf(letter) >= 0

var upper_case = function(letter){
    return letter === letter.toUpperCase() && uppers.indexOf(letter) >= 0

You can test if your array has an upper case or lower case string by using the match method and regex, below is just a basic foundation to start your test

  var array = ['a', 'b', 'c', 'A', 'B', 'C', '(', ')', '+', '-', '~', '*'];
  var character = array.join('')

  var test = function(search){
      upperCase = search.match(/[A-Z]/g)

      lowerCase = search.match(/[a-z]/g)


There's a really simple answer, which nobody else has mentioned:

function isLowerCase(str) {
    return str !== str.toUpperCase();

If str.toUpperCase() does not return the same str, it has to be lower case. To test for upper case you change it to str !== str.toLowererCase().

Unlike some other answers, it works correctly on non-alpha characters (returns false) and it works for other alphabets, accented characters etc.

You can also use this, it will check the string for lower and uppercase

var s = "a"
  alert ('lower case true');

if(/[A-Z]/.test(s)) {
 alert ('upper case true'); 

function isUpperCase(myString) { 
  return (myString == myString.toUpperCase()); 
function isLowerCase(myString) { 
  return (myString == myString.toLowerCase());