问通过JavaScript过滤ul列表没有正确过滤列表

EN

<input type="text" id="myMovies" onkeyup="searchTitle()" placeholder="Is your movie on here?">
        <ul id="movieList">
            <li>item 1</li>
            <li>item 2</li>
            <li>item 3</li>
            <li>item 4</li>
        </ul>

function searchTitle(){
    var input, filter,  ul, li, a, i;
    input = document.getElementById("myMovies");
    filter = input.nodeValue.toUpperCase();
    ul = document.getElementById("movieList");
    li = ul.getElementsByTagName("li");

    for (i = 0; i < li.length; i++){
        a = li[i].getElementsByTagName("a"[0]);
        if (a.innerHTML.toUpperCase().indexof(filter) > -1) {
            li[a].style.display = "";
        }
        else{
            li[a].style.display = "Not found";
        }
    }
}