4-5 例子

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>