JSTreeView

var anchorTags = document.getElementsByTagName('a');
var lenA = anchorTags.length;

//first we hide all nested lists and attach onlcick event to all 'a' tags
(function () {
    var nestedLists = document.querySelectorAll('ul ul');
    var len = nestedLists.length;

    for (var i = 0; i < len; i++) {
        nestedLists[i].style.display = "none";
    };

    for (var i = 0; i < lenA; i++) {
        anchorTags[i].addEventListener('click', expandOrCollapse);
    };
}())

//no we define the function that hides/shows the nested lists
function expandOrCollapse() {
    if (this.nextElementSibling.style.display == 'none') {
        this.nextElementSibling.style.display = 'block';
    }
    else if (this.nextElementSibling.style.display == 'block') {
        this.nextElementSibling.style.display = 'none';
    }
}