Check if a string is uppercase or lowercase in JavaScript

It's not uncommon to need to check if a string is uppercase or lowercase in JavaScript. Fundamentally, this is a very simple task, as we can easily convert any string to uppercase or lowercase. We can then compare the original string to the converted string and get the desired result.

Check if a string is uppercase

In order to check if a string is uppercase, we can convert the string to uppercase using String.prototype.toUpperCase() and compare it to the original string.

const isUpperCase = str => str === str.toUpperCase();

isUpperCase('ABC'); // true
isUpperCase('A3@$'); // true
isUpperCase('aB4'); // false

Check if a string is lowercase

Conversely, we can do the same for lowercase strings, comparing the original string with the output of String.prototype.toLowerCase().

const isLowerCase = str => str === str.toLowerCase();

isLowerCase('abc'); // true
isLowerCase('a3@$'); // true
isLowerCase('Ab4'); // false

Dealing with non-alphabetic characters

The above examples work well for alphabetic characters, but what about non-alphabetic characters? For example, '!@#$' is neither uppercase nor lowercase, but both isUpperCase('!@#$') and isLowerCase('!@#$') return true.

Regular expressions offer a solution to this problem. Here's a quick rundown of the regular expression syntax we'll be using:

• Use the ^ anchor to match the start of the string.
• Use [a-z] or [A-Z] to match a range of alphabetic characters (case-sensitive).
• Use a positive lookahead (?=) to ensure that at least one alphabetic character is present in the string.
• Use \s to allow whitespace characters (optional).
• Use the + quantifier to match one or more of the preceding token.
• Use the $ anchor to match the end of the string.

After setting up the regular expression, we can use RegExp.prototype.test() to check if the string matches.

const isUpperCase = str => /^(?=[A-Z])[A-Z\s]+$/.test(str);

isUpperCase('ABC'); // true
isUpperCase('A BC'); // true
isUpperCase('A3@$'); // false
isUpperCase(' '); // false
isUpperCase('!@#$'); // false

const isLowerCase = str => /^(?=[a-z])[a-z\s]+$/.test(str);

isLowerCase('abc'); // true
isLowerCase('a bc'); // true
isLowerCase('a3@$'); // false
isLowerCase(' '); // false
isLowerCase('!@#$'); // false

[!TIP]

If you don't want to allow whitespace characters, the regular expressions for uppercase and lowercase can be simplified to /^[A-Z]+$/ and /^[a-z]+$/ respectively.