How to check if character is a letter in Javascript?

There my favorite and reliable methods to check if a character is a letter.

Using ASCII Code Values

Another common method is checking ASCII code values using charCodeAt():

function isLetter(char) {
    const code = char.charCodeAt(0);
    return (code >= 65 && code <= 90) ||  // uppercase letters
           (code >= 97 && code <= 122);   // lowercase letters
}

console.log(isLetter('A')); // true
console.log(isLetter('z')); // true
console.log(isLetter('5')); // false

Why it works ASCII

Using String Methods

A simple but less performant method uses a combination of toUpperCase() and toLowerCase():

function isLetter(char) {
  return char.toLowerCase() !== char.toUpperCase();
}

console.log(isLetter('A')); // true
console.log(isLetter('!')); // false

This method is the most marvelous 🦄

Letters have different uppercase and lowercase versions, while non-letters (like numbers and special characters) remain unchanged when using these case conversion methods.