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++;
}