<input type="text" id="txt" placeholder="输入要添加的文本" />
    <button id="btn">加 </button>
    <ul id="ul">
        <li>11111</li>
        <li>22</li>
        <li>3333</li>
        <li>4444</li>
    </ul>
    <script type="text/javascript">
        var ul = document.getElementById("ul");
        var lis = ul.getElementsByTagName('li');
        var btn = document.getElementById("btn");
        
        btn.onclick = function() {   //动态添加li
            var txt = document.getElementById("txt"),
                txtValue = txt.value,
                ali = document.createElement("li");
                console.log(txt.value);                    
                ali.innerHTML = txtValue;
                ul.appendChild(ali);
        }
        
        ul.onmouseover = function(ev) {
            var ev = ev || window.event;   //获取发生事件 event 兼容      =====1
            var target = ev.target || ev.srcElement;   //获取真正被触发的元素     =====2
            if (target.nodeName.toLocaleLowerCase() == 'li') {
                //判断target是否是所需要的元素  正是因为这个判断 我们可以得到任何想要的元素  a li td 等等
                target.style.background = "red";
            }
        }
        ul.onmouseout = function(ev) {
            var ev = ev || window.event;
            var target = ev.target || ev.srcElement;
            if (target.nodeName.toLocaleLowerCase() == 'li') {
                target.style.background = "";
            }
        }
    </script>

Comment comprendre l'écriture des codes marqués 1 et 2 ? Je ne comprends pas = =D'où vient l'API