<li>1</li>
<li>2</li>
<li>3</li>
<li>4</li>

var oBtn = document.getElementsByTagName('button')[0],
    oList = document.getElementsByTagName('ul')[0],
    oLi = oList.getElementsByTagName('li');
oBtn.onclick = function () {
    var li = document.createElement('li');
    li.innerText = oLi.length + 1;
    oList.appendChild(li);
}
oList.onclick = function (ev) {
    var ev = ev || window.event,
        tar = ev.target || ev.srcElement;
    // tar 即为被点击的 li 元素
    console.log(tar.innerHTML); 
    // 返回在所有兄弟元素中的索引，借用数组 indexOf 方法
    console.log(Array.prototype.indexOf.call(oLi, tar));
}