原生JavaScript是怎样实现渐隐效果的

function id(name) {
    return document.getElementById(name);
}
//遍历函数
function each(arr, callback) {
    if (arr.forEach) {
        arr.forEach(callback);
    } else {
        for (var i = 0,
        len = arr.length; i < len; i++) callback.call(this, arr[i], i, arr);
    }
}
function fadeIn(elem) {
    setOpacity(elem, 0) for (var i = 0; i < 20; i++) { (function() {
            var pos = i * 5;
            setTimeout(function() {
                setOpacity(elem, pos)
            },
            i * 25);
        })(i);
    }
}
function fadeOut(elem) {
    for (var i = 0; i <= 20; i++) { (function() {
            var pos = 100 - i * 5;
            setTimeout(function() {
                setOpacity(elem, pos)
            },
            i * 25);
        })(i);
    }
}
// 设置透明度
function setOpacity(elem, level) {
    if (elem.filters) {
        elem.style.filter = "alpha(opacity=" + level + ")";
    } else {
        elem.style.opacity = level / 100;
    }
}