原生JavaScript实现简易计算器源码与解析

<!DOCTYPE html>
<html>
    <head>
        <meta charset="utf-8">
        <title>计算器</title>
        <style>
            #b1{
                text-align: center;
                background-color: lightblue;
                width: 650px;
                height: 650px;
                border: 1px solid black;
                margin-left: 400px;
                border-radius: 18px;

            }
            #b2{
                text-align: center;
                width: 300px;
                height: 80px;
                margin-top: 30px;
                font-family: "微软雅黑";
                font-size: 20px;
                border-radius: 8px;
                outline:none;
            }
            ul{
                list-style-type: none;

            }
            li{
                float: left;
                margin-left: 40px;
                margin-top: 30px;
            }
            li input{
                width: 100px;
                height: 50px;
                border-radius: 8px;
                font-family: "微软雅黑";
                font-size: 20px;
                outline:none;
            }
        li input:hover{
                background-color:red ;
            }
        </style>

        <script>
        var beforeNum=0;
        function addNum(a){
             beforeNum=document.getElementById("b2").value;
             var nowNum=beforeNum;
            if(beforeNum!=0){
             nowNum=beforeNum+a;    
            }else{
                if(beforeNum.substring(beforeNum.length-2,beforeNum.length-1)=="."){
                    nowNum=beforeNum+a; 
                }
                 if(a!="+"&&a!="-"&&a!="*"&&a!="/"&&(beforeNum.substring(beforeNum.length-2,beforeNum.length-1)!=".")){
                 nowNum=a;  
                }
            }
            document.getElementById("b2").value=nowNum;
        }


        function jisuan(){
            document.getElementById("b2").value=eval(document.getElementById("b2").value);
        }

        function clearNum(){
            document.getElementById("b2").value=0;
        }

        function backAgain(){
            beforeNum=document.getElementById("b2").value;
            if(beforeNum.length!=1){
            document.getElementById("b2").value=beforeNum.substring(0,beforeNum.length-1);
            }else if(beforeNum.length==1){
                document.getElementById("b2").value=0;
            }
        }
        function addPoint(b){
            beforeNum=document.getElementById("b2").value;
            if((beforeNum.indexOf(".")!=beforeNum.length-1)){
                document.getElementById("b2").value=beforeNum+b;
            }
        }
        function offCl(){
            document.getElementById("b2").value="";
        }
        </script>
    </head>
    <body>
        <div id="b1">
            <p style="font-size: 20px;">在线简易计算器</p>
            <div><input id="b2" name="wenben" value="0"/></div>
            <div>
                <ul>

                    <li><input type="button" value="1" onclick="addNum(this.value);"/></li>
                    <li><input type="button" value="2" onclick="addNum(this.value);"/></li>
                    <li><input type="button" value="3" onclick="addNum(this.value);"/></li>
                    <li><input type="button" value="4" onclick="addNum(this.value);"/></li>
                    </ul>
                <ul>
                    <li><input type="button" value="5" onclick="addNum(this.value);"/></li>
                    <li><input type="button" value="6" onclick="addNum(this.value);"/></li>
                    <li><input type="button" value="7" onclick="addNum(this.value);"/></li>
                    <li><input type="button" value="8" onclick="addNum(this.value);"/></li>
                    </ul>
                <ul>
                    <li><input type="button" value="9" onclick="addNum(this.value);"/></li>
                    <li><input type="button" value="0" onclick="addNum(this.value);"/></li>
                    <li><input type="button" value="+" onclick="addNum(this.value);"/></li>
                    <li><input type="button" value="-" onclick="addNum(this.value);"/></li>
                </ul>
                    <ul>
                    <li><input type="button" value="*" onclick="addNum(this.value);"/></li>
                    <li><input type="button" value="/" onclick="addNum(this.value);"/></li>
                    <li><input type="button" value="." onclick="addPoint(this.value);"/></li>
                    <li><input type="button" value="=" onclick="jisuan();"/></li>
                </ul>
                </ul>
                    <ul>
                    <li><input type="button" value="回退" onclick="backAgain();"/></li>
                    <li><input type="button" value="清除" onclick="clearNum();"/></li>
                    <li><input type="button" value="关机" onclick="offCl();"/></li>
                </ul>
            </div>
        </div>
    </body>
</html>