FCC BAS #7 - Bonfire: Title Case a Sentence

function titleCase(str) {
    str = str.toLowerCase().split(" ");
    var word = [];
    for(x=0; x<str.length; x++) {
        var a = str[x].charAt(0).toUpperCase();
        var b = str[x].substr(1);
        word.push(a + b);
    } 
    
    str = word.join(' ');
    return str;
}
 
titleCase("I'm a little tea pot");