Javascript 获取验证码60秒倒计时

<button id="time">获取验证码</button>
var countdown = 60;
var but = document.getElementById('time');
but.addEventListener('click', function (e) {
    setTime(this);
})

function setTime (elem) {
    if (countdown === 0) {
        elem.innerHTML = "获取验证码";
        countdown = 5;
        elem.disabled = false;
        clearTimeout(anima);
    } else {
        countdown--;
        elem.innerHTML = "重新获取(" + countdown + ")";        
        elem.disabled = true;
        var anima = setTimeout(function () {
            setTime(elem);
        }, 1000);
    }
}