SOURCE

var ipt = document.querySelector('input');
var btn = document.querySelector('button');
var time = 5;
btn.addEventListener('click',function(){
    btn.disabled = true;
	var timer = setInterval(function(){
        btn.innerHTML =  '还剩下'+time+'秒';
        time--;
       if(time == 0){
        clearInterval(timer)
        btn.disabled = false;
        btn.innerHTML = '发送';
        time = 5;
    } 
    },1000)
    
})

手机号码：<input type="text"></input>
<button>发送</button>