1、获取父元素

obj.parentElement

示例: input鼠标移动到眼睛就显示密码，否则隐藏密码
html代码

<div class="col-sm-10">
    <input style="float: left" type="password" class="form-control" id="old_pwd" placeholder="请输入旧密码">
    <span style="float: right;margin-top:-33px;font-size:17px;padding: 8px" class="fa fa-eye mouse-eye"></span>
</div>

js代码

    window.onload = function() {
        let mouseEyeEles = document.getElementsByClassName("mouse-eye");
        for (let o = 0; o < mouseEyeEles.length; o++) {
            mouseEyeEles[o].onmouseover = function() {
                this.parentElement.getElementsByTagName("input")[0].type="text";
            }
            mouseEyeEles[o].onmouseout = function() {
                this.parentElement.getElementsByTagName("input")[0].type="password";
            }
        }
    }