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

Question

The answer by josh and maleki will return true on both upper and lower case if the character or the whole string is numeric. making the result a false result. example using josh

var character = '5';
if (character == character.toUpperCase()) {
 alert ('upper case true');
}
if (character == character.toLowerCase()){
 alert ('lower case true');
}

another way is to test it first if it is numeric, else test it if upper or lower case example

var strings = 'this iS a TeSt 523 Now!';
var i=0;
var character='';
while (i <= strings.length){
    character = strings.charAt(i);
    if (!isNaN(character * 1)){
        alert('character is numeric');
    }else{
        if (character == character.toUpperCase()) {
            alert ('upper case true');
        }
        if (character == character.toLowerCase()){
            alert ('lower case true');
        }
    }
    i++;
}

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

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:

assert(isLowerCase("a"))
assert(!isLowerCase("Ü"))
assert(!isLowerCase("4"))
assert(!isLowerCase("_"))

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

This will log true if character is uppercase letter, and log false in every other case:

var letters = ['a', 'b', 'c', 'A', 'B', 'C', '(', ')', '+', '-', '~', '*'];

​​​for (var ​i = 0; i<letters.length; i++) {
    if (letters[i] === letters[i].toUpperCase()
        && letters[i] !== letters[i].toLowerCase()) {
        console.log(letters[i] + ": " + true);
    } else {
        console.log(letters[i] + ": " + false);
    }
}​

You may test it here: http://jsfiddle.net/Axfxz/ (use Firebug or sth).

​​​for (var ​i = 0; i<letters.length; i++) {
    if (letters[i] !== letters[i].toUpperCase()
        && letters[i] === letters[i].toLowerCase()) {
        console.log(letters[i] + ": " + true);
    } else {
        console.log(letters[i] + ": " + false);
    }
}​

and this is for lowercase:).

const isUpperCase = (string) => /^[A-Z]*$/.test(string)

then :

isUpperCase('A') // true
isUpperCase('a') // false

You could utilize a regular expression test and the toUpperCase method:

String.prototype.charAtIsUpper = function (atpos){
      var chr = this.charAt(atpos);
      return /[A-Z]|[\u0080-\u024F]/.test(chr) && chr === chr.toUpperCase();
};
// usage (note: character position is zero based)
'hi There'.charAtIsUpper(3);      //=> true
'BLUE CURAÇAO'.charAtIsUpper(9);  //=> true
'Hello, World!'.charAtIsUpper(5); //=> false

See also