vowels consonants javascript

//Ask to the user to type a letter
function readLetter() {
    return prompt("Type a letter: ");
}

//Defines true if is a vowel(uppercase or lowercase);
function isVowel(character) {
    switch (character) {
        case "a":
        case "A":
        case "e":
        case "E":
        case "i":
        case "I":  
        case "o":
        case "O":
        case "u":
        case "U": 
            return true;
        default:
            return false;
    }
}

// Brings a dependent message if previous result is true or false.
var letter = readLetter();
if (isVowel(letter)) alert("The letter is a vowel.");
else alert("The letter is consonant.");