JavaScript: check if a giving string contains only letters or digits

Using regexp, you can add 0-9 to say any digit between 0 and 9:

boolean onlyLettersAndDigits(String str) {
      return str.matches("^[a-zA-Z0-9]+$");
    }

Add 0-9 also to your regex

 boolean onlyLetters(String str) {
   return str.match("^[A-Za-z0-9]+$");
 }