给网站加上全局字体增大减小功能(自适应) JavaScript

<div id="fontSizeDiv">
    <span id="Jia">&nbsp;+&nbsp;</span>
    <span id="Jian">&nbsp;-&nbsp;</span>
    <span id="Guan">关闭</span>

    <span id="fontSize">...</span>
</div>

div#fontSizeDiv {
    margin: 10% 0;
    width: auto;
    padding: 6px 10px;
    display: inline-block;
    background-color: #eeeeee;
    position: fixed;
    top: 5%;
    right: 0;
}

span#Jia {
    user-select: none;
    cursor: pointer;
    display: inline-block;
    width: 50px;
    font-size: 20px !important;
    line-height: 26px !important;
    background-color: #dddddd;
    border-radius: 2px;

    transition: all 100ms linear;
}
span#Jia:hover {
    box-shadow: 1px 1px 1px #808080;

    transition: all 100ms linear;
}

span#Jian {
    user-select: none;
    cursor: pointer;
    display: inline-block;
    width: 50px;
    font-size: 20px !important;
    line-height: 26px !important;
    border-radius: 2px;

    transition: all 100ms linear;
}
span#Jian:hover {
    background-color: #dddddd;
    transition: all 100ms linear;
}

span#fontSize {
    display: block;
    margin-top: 4px;
    user-select: none;
    cursor: pointer;
    font-size: 20px !important;
    line-height: 26px !important;
}

span#Guan {
    user-select: none;
    cursor: pointer;
    display: inline-block;
    width: 50px;
    font-size: 20px !important;
    line-height: 26px !important;
    border-radius: 2px;

    transition: all 100ms linear;
}
span#Guan:hover {
    background-color: #dddddd;

    transition: all 100ms linear;
}

// 变量
var Body;
var Jia;
var Jian;
var fontSize;
var fontSizeDiv;

var numberJia;
numberJia = 18;

var numberJian;
numberJian = 0;

// 获取全局字体大小并显示
fontSize = document.getElementById('fontSize');
fontSize.innerHTML = window.getComputedStyle(document.body).getPropertyValue('font-size');

// 增加 按钮点击事件
Jia = document.getElementById('Jia').onclick = function () {
    if (numberJia < 50) {
        numberJia ++;

        Body = document.getElementById('body').style.fontSize = numberJia + 'px';
        fontSize.innerHTML = window.getComputedStyle(document.body).getPropertyValue('font-size');
    }
}

// 减少 按钮点击事件
Jian = document.getElementById('Jian').onclick = function () {
    if (numberJia > 0) {
        numberJia --;

        Body = document.getElementById('body').style.fontSize = numberJia + 'px';
        fontSize.innerHTML = window.getComputedStyle(document.body).getPropertyValue('font-size');
    }
}

// 关闭 按钮点击事件
document.getElementById('Guan').onclick = function () {
    fontSizeDiv = document.getElementById('fontSizeDiv').style.display = 'none';
}