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

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
}

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:).