循环执行此函数，为什么获取的值没有被累加

Question

想要通过不断执行定时器，改变元素的opacity,但是只函数每次获取的元素的opacity都是0，没有被累加；代码如下：function appear() {   var opity = document.getElementById('fade-obj').style.opacity;     console.log(document.getElementById('fade-obj').style.opacity);        if(opity != 1) {           document.getElementById('fade-btn').disabled = 'true';                  opity += 0.2;        document.getElementById('fade-obj').style.opacity = opity;        setTimeout(appear,500);    }else{        document.getElementById('fade-btn').innerHTML = '淡出';        document.getElementById('fade-btn').removeAttribute('disabled');    }}document.getElementById('fade-btn').addEventListener('click', function() {      if (document.getElementById('fade-btn').innerHTML == '淡入') {        setTimeout(appear,500);    }}, false)结果如下：不知道为什么document.getElementById('fade-obj').style.opacity的值不变