Check if the letter is uppercase in JavaScript

Published: · Reading time: 6 min

While working with strings, especially when validating forms, you may want to check whether the letter is uppercase.

By applying a few methods you can check almost any character’s case.

To find the letter in a string you can use one of the multiple ways, but for the sake of the demonstration, let’s specify the string in a variable.

String.toUpperCase() and String.toLowerCase() method

The String.toUpperCase() method will convert a string to uppercase, after that you can compare it to your test letter.

const charA = "W"
const charB = "b"

console.log(charA.toUpperCase()) // W
console.log(charA.toUpperCase() === charA) // true

console.log(charB.toUpperCase()) // B
console.log(charB.toUpperCase() === charB) // false

However there’s a downside to this approach. If the character happens to be a number, punctuation or special character then it would not work.

The toUpperCase() method would just return a given value (number or punctuation), so the condition would be true in these cases.

const charA = "3"
const charB = ";"

console.log(charA.toUpperCase()) // "3"
console.log(charA.toUpperCase() === charA) // true

console.log(charB.toUpperCase()) // ";"
console.log(charB.toUpperCase() === charB) // true

To handle this issue we need to extend the condition and check whether the character has an uppercase and lowercase variant by using the toLowerCase method.

const charA = "3"
const charB = ";"

console.log(charA.toUpperCase() === charA && charA.toLowerCase() !== charA) // false
console.log(charB.toUpperCase() === charB && charB.toLowerCase() !== charB) // false

This condition will work for almost any letter in any language. You can wrap it into a function and pass a character as a parameter:

const isUpperCase = (character) => {
  return character.toUpperCase() === character && character.toLowerCase() !== character

isUpperCase("W") // true
isUpperCase("b") // false
isUpperCase("3") // false
isUpperCase(";") // false
isUpperCase("ç") // false
isUpperCase("Ç") // true
isUpperCase("г") // false
isUpperCase("Г") // true

Checking against a regular expression

Another approach would be to match a string against a regular expression. You can use test() or match() methods to check if the letter is uppercase.

The regular expression must include all capital letters of the alphabet within a character set, so in our case it will be /[A-Z]/.

To get the boolean value you can use the test() method, it will test a string against a regular expression and will return true if at least one character is uppercase, otherwise it will return false.

const regExp = /[A-Z]/

console.log(regExp.test("a")) // false
console.log(regExp.test("A")) // true
console.log(regExp.test("3")) // false
console.log(regExp.test(";")) // false
console.log(regExp.test("abCdefg1234?!,.;")) // true
console.log(regExp.test("abcdefg1234?!,.;")) // false

The match method will return an array if character is matched against a regular expression, otherwise it will return null.

const regExp = /[A-Z]/

console.log("a".match(regExp)) // null
console.log("3".match(regExp)) // null
console.log(";".match(regExp)) // null
console.log("A".match(regExp)) // ['A', index: 0, input: 'A', groups: undefined]

To handle letters from languages other than English for both test and match methods, you’ll need to extend your regular expression by specifying the first and the last letter of each alphabet you’re going to check.

const regExp = /[A-ZÀ-ÖА-Я]/
Like this article? Share it on: