Javascript: find longest word in a string

function longestWord(string) {
    var str = string.split("");
    var longest = 0;
    var word = null;
    for (var i = 0; i < str.length - 1; i++) {
        if (longest < str[i].length) {
            longest = str[i].length;
            word = str[i];
        }
    }
    return word;
}

When I call longestWord("Pride and Prejudice"), it returns 'Pride' and not 'Prejudice' which is the longest word... why? I checked some other similar questions, but the solutions looked a lot like my code.