SOURCE

 var i;
var conduct = document.getElementById('conduct');
var num = 0,
    result = 0,
    numshow = "0";
var operate = 0; //判断输入状态的标志 0,1 0为可以连续输入状态 1为重新输入状态
var calcul = 0; //判断计算状态的标志 
var calculator = document.getElementsByClassName('screen');

function jisuan(num) {
    var str = String(calculator[0].value); //获得当前显示数据 
    str = (str != "0") ? ((operate == 0) ? str : "") : ""; 
    //如果当前值不是"0"，且状态为0，则返回当前值，否则返回空值; 
    str = str + String(num); //给当前值追加字符 
    calculator[0].value = str;
    operate = 0; //重置输入状态  
}

function del() { //退格 
    var str = String(calculator[0].value);
    str = (str != "0") ? str : "";
    str = str.substr(0, str.length - 1);
    str = (str != "") ? str : "0";
    calculator[0].value = str;
}

function clearscreen() { //清除数据 
    num = 0;
    result = 0;
    numshow = "0";
    calculator[0].value = "0";
}
function dot(){ 
var str=String(calculator[0].value); 
str=(str!="0") ? ((operate==0) ? str : "0") : "0"; //如果当前值不是"0"，且状态为0，则返回当前值，否则返回"0"; 
for(i=0; i<=str.length;i++){ //判断是否已经有一个点号 
if(str.substr(i,1)==".") return false; //如果有则不再插入 
} 
str=str + "."; 
calculator[0].value=str; 
operate=0; 
}
function dzero(){
    var str=String(calculator[0].value); 
str=(str!="0") ? ((operate==0) ? str : "0") : "0"; //如果当前值不是"0"，且状态为0，则返回当前值，否则返回"0"; 
str=str+"00";
calculator[0].value =str;
operate = 0;
}
function plus() { //加法 
    calculate(); //调用计算函数 
    operate = 1; //更改输入状态 
    calcul = 1; //更改计算状态为加 
}

function minus() { //减法 
    calculate();
    operate = 1;
    calcul = 2;
}

function times() { //乘法 
    calculate();
    operate = 1;
    calcul = 3;
}

function divide() { //除法 
    calculate();
    operate = 1;
    calcul = 4;
}

function persent() { //求余 
    calculate();
    operate = 1;
    calcul = 5;
}

function equal() {
    calculate(); //等于 
    operate = 1;
    num = 0; //初始化
    result = 0; //初始化
    numshow = "0"; //初始化
}

function calculate() {
    numshow = Number(calculator[0].value);
    if (num != 0) { //判断前一个运算数是否为零 
        switch (calcul) { //判断要输入状态 
            case 1:
                result = num + numshow;
                break; //计算"+" 
            case 2:
                result = num - numshow;
                break; //计算"-" 
            case 3:
                result = num * numshow;
                break;
            case 4:
                result = num / numshow;
                break;
            case 5:
                result = num % numshow;
                break;
        }
    } else {
        result = numshow;
    }
    numshow = String(result);
    calculator[0].value = numshow;
    num = result; //存储当前值 
}

  <div id="complex">
        <div id="calcu-screen">
        <input type="text" name="numScreen" class="screen" value="0" onfocus="this.blur();" /> 
        </div> 
        <table id="suanpan" cellspacing="0">
            <tr class="graybgc">
                <td style="color:#FF4500" onclick="clearscreen()">AC</td>
                <td style="color:#FF4500" onclick="del();">←</td>
                <td onclick="persent()">%</td>
                <td class="bgcg" onclick="divide()">÷</td>
            </tr>
            <tr>
                <td onclick="jisuan(7);">7</td>
                <td onclick="jisuan(8);">8</td>
                <td onclick="jisuan(9);">9</td>
                <td class="bgcg" onclick="times()">×</td>
            </tr>
            <tr>
                <td onclick="jisuan(4);">4</td>
                <td onclick="jisuan(5);">5</td>
                <td onclick="jisuan(6);">6</td>
                <td class="bgcg" onclick="minus()">-</td>
            </tr>
            <tr>
                <td onclick="jisuan(1);">1</td>
                <td onclick="jisuan(2);">2</td>
                <td onclick="jisuan(3);">3</td>
                <td class="bgcg" onclick="plus()">+</td>
            </tr>
            <tr>
                <td onclick="jisuan(0);">0</td>
                <td onclick="dzero()">00</td>
                <td onclick="dot()">.</td>
                <td class="bgcg" onclick="equal()">=</td>
            </tr>
        </table>
    </div>

       #suanpan{
    border-top: 1px solid #f8f8f8;
    border-left: 1px solid #f8f8f8;
}
#suanpan td{
    border-right: 1px solid #f8f8f8;
    border-bottom: 1px solid #f8f8f8;
    margin:0;
    text-align: center;
    width: 40px;
    height: 40px;
    font-weight: "微软雅黑";
    cursor: pointer;
}
.bgcg{
    background: #f1f1f1;
}
.graybgc{
    background: #f1f1f1;
}
#suanpan td:hover{
    background: #f6f6f6;
}
#suanpan td:active{
    background: #C0C0C0;
}
.screen{
    min-height: 20px;
    width: 169px;
    background: #f5f5f5;
    text-align: right;
    overflow: hidden;
    margin:0;
}
#calcu-screen{
    max-height: 40px;
    font-weight: bold;
    font-family: "微软雅黑";
    font-size: 18px;
    max-width: 160px;
    word-break: break-all;
    margin:0;
}
#complex{
    margin: 0 auto;
    width: 250px;
    margin-top: 50px;
}