例子A

1-1 3s倒计时

<div id="app">
        3
    </div>
    <script>
        var num =3;
        var app = document.getElementById("app");
        // function go(){
        //     setTimeout(()=>{
        //         num--;
        //         if(num>=0){
        //             go();
        //             app.innerHTML = num;
        //         }
        //     },1000)
        // }
        // go();
        var timer = setInterval(()=>{
            num--;
            console.log(num)
            if(num>=0){
                app.innerHTML = num;
            }else{
                clearInterval(timer)
            }
        },1000)
    </script>

1-2按钮5s倒计时

<button id="btn">发送验证码</button>
    <script>
        var num =3;
        var btn = document.getElementById("btn");
        var timer;
        btn.onclick = function(){
            btn.disabled = true;
            btn.innerHTML = num;
            timer = setInterval(()=>{
                num--;
                if(num>=0){
                    btn.innerHTML = num;
                }else{
                    btn.innerHTML = "发送验证码";
                    clearInterval(timer)
                    btn.disabled = false;
                    num=3;
                }
            },1000)
        }
    </script>

1-3 点击对应按钮,对应按钮删除

点击删除按钮，点击确定删除对应的元素

<ul>
  <li>html <button>删除</button></li>
    <li>css <button>删除</button></li>
      <li>js  <button>删除</button></li>
        </ul>            
var btns = document.getElementsByTagName("button")
        for(var i=0;i<btns.length;i++){
            btns[i].onclick = function(){
                var temp = window.confirm("确定吗")
                if(temp){
                    this.parentNode.style.display = "none"
                }
            }
        }