html

<input type="button" id="btn" value="免費獲取驗證碼" onclick="settime(this)" />

js

// 短信驗證碼倒計時
var countdown=60;
function settime(obj) {
    if (countdown == 0) {
        obj.removeAttribute("disabled");
        obj.value="免費獲取驗證碼";
        countdown = 60;
        return;
    } else {
        obj.setAttribute("disabled", true);
        obj.value="重新發送(" + countdown + ")";
        countdown--;
    }
    setTimeout(function() {
            settime(obj) }
        ,1000)
}

js 發送短信驗證碼倒計時