JAVASCRIPT实现简单计算器

最终效果如下图-2，有bug：就是整数后点击%号结果正确，如果小数后面点击%的话结果就错误！其他都正常，求指点：input的value是string类型的，在JS中改如何正确处理下图-1中的if部分？？

图-1

图-2

HTML代码如下

<body>
<div id="calculator">
    <div class="LOGO">
        <span class="name">简单的计算器</span>
        <span class="verson">@walker</span>
    </div>
    <div id="shuRu">
        <!--screen输入栏-->
        <div class="screen">
            <input type="text" id="screenName" name="screenName" class="screen">
        </div>
    </div>
    <div id="keys">
        <!-- j -->
        <!--第一排-->
        <input type="button" id="7" onclick="jsq(this.id)" value="7" class="buttons">
        <input type="button" id="8" onclick="jsq(this.id)" value="8" class="buttons">
        <input type="button" id="9" onclick="jsq(this.id)" value="9" class="buttons">
        <input type="button" id="Back" onclick="tuiGe()" value="Back" class="buttons">
        <input type="button" id="C" onclick="clearNum()" value="C" class="buttons" style="margin-right:0px">
        <!--第二排-->
        <input type="button" id="4" onclick="jsq(this.id)" value="4" class="buttons">
        <input type="button" id="5" onclick="jsq(this.id)" value="5" class="buttons">
        <input type="button" id="6" onclick="jsq(this.id)" value="6" class="buttons">
        <input type="button" id="*" onclick="jsq(this.id)" value="X" class="buttons">
        <input type="button" id="/" onclick="jsq(this.id)" value="/" class="buttons" style="margin-right:0px">
        <!--第三排-->
        <input type="button" id="1" onclick="jsq(this.id)" value="1" class="buttons">
        <input type="button" id="2" onclick="jsq(this.id)" value="2" class="buttons">
        <input type="button" id="3" onclick="jsq(this.id)" value="3" class="buttons">
        <input type="button" id="+" onclick="jsq(this.id)" value="+" class="buttons">
        <input type="button" id="-" onclick="jsq(this.id)" value="-" class="buttons" style="margin-right:0px">
        <!--第四排-->
        <input type="button" id="0" onclick="jsq(this.id)" value="0" class="buttons">
        <input type="button" id="00" onclick="jsq(this.id)" value="00" class="buttons">
        <input type="button" id="." onclick="jsq(this.id)" value="." class="buttons">
        <input type="button" id="%" onclick="jsq(this.id)" value="%" class="buttons">
        <input type="button" id="eva" onclick="eva()" value="=" class="buttons" style="margin-right:0px">
    </div>
    <div class="footer">
        <span class="aside">欢迎使用JavaScript计算器</span>
            <span class="link">
                <a href="#" title="声明" target="_blank">反馈</a>
            </span>
    </div>
</div>
</body>

CSS代码如下：

<style>
        /*Basic reset*/
*{
    margin:;
    padding:;
    box-sizing: border-box;
    font:  14px Arial,sans-serif;
}
html{
    height:100%;
    background-color:lightslategrey;
}

#calculator{
    margin: 15px auto;
    width:330px;
    height:400px;
    border: 1px solid lightgray;
    background-color:darkgrey;
    padding:15px;
}

/*LOGO*/
.LOGO{
    height:20px;

}
.LOGO .name{
    float:left;
    line-height:30px;
}
.LOGO .verson{
    float:right;
    line-height:30px;
}
/*screen*/
#shuRu{
    margin-top:15px;
}
.screen{
    margin-top:5px;
    width:300px;
    height:40px;
    text-align: right;
    padding-right:10px;
    font-size:20px;
}
#keys{
    border:1px solid lightgray;
    height:223px;
    margin-top:25px;
    padding:8px;
}
#keys .last{
    margin-right:0px;
}
.footer{
    margin-top:20px;
    height:20px;
}
.footer .link{
    float:right;
}

#keys .buttons{
    float:left;
    width: 42px;
    height: 36px;
    text-align:center;
    background-color:lightgray;
    margin: 0 17px 20px 0;
}
    </style>

javascript代码如下：

<script>
        var num = 0;  // 定义第一个输入的数据
        function jsq(num) {
            //获取当前输入
			if(num=="%"){
				document.getElementById('screenName').value=Math.round(document.getElementById('screenName').value)/100;
			}else{
				document.getElementById('screenName').value += document.getElementById(num).value;
			}
        }
        function eva() {
            //计算输入结果
            document.getElementById("screenName").value = eval(document.getElementById("screenName").value);
        }
        function clearNum() {
            //清0
            document.getElementById("screenName").value = null;
            document.getElementById("screenName").focus();
        }
        function tuiGe() {
            //退格
            var arr = document.getElementById("screenName");
            arr.value = arr.value.substring(0, arr.value.length - 1);
        }
    </script>