注意：该计算器只能进行个位数的加减乘除
思路要点：（1）在读取按键的数字的时候，通过 :for循环下的 集合【i】.onclick=function(){ 变量=this.innerHTML}来取得，必须要使用“this”来代替 “被取集合【i】”，否则无法取到值，应该是由于“集合【i】”被设置了onclick=function函数的原因。
（2）然后在取到按键数字的同时判断按键次数，创造变量接收第一次，第二次，第三次取到的字符。同时判断第二次取到的加减乘除号，分情况进行计算。
<!DOCTYPE html>
<html>
<head>
<meta charset="UTF-8">
<title>简易计算器</title>
<style type="text/css">
*{
margin: 0 auto;
padding: 0;
}
#bigk{
320px;
height: 450px;
background-color: gainsboro;
border-radius: 8px;
box-shadow: 5px 5px 5px darkgray;
margin-top: 30px;
border: 1px solid black;
overflow: hidden;
}
#sck{
312px;
height: 30px;
border: 1px solid black;
margin-top: 10px;
background-color: gainsboro;
border-radius: 4px;
}
.hengk{
312px;
height:100px ;
}
.xiaok{
float: left;
70px;
height: 90px;
border: 1px solid black;
margin-top: 5px;
margin-left: 5px;
font-size: 24px;
font-weight: bolder;
text-align: center;
line-height: 90px;
border-radius: 4px;
}
.xiaok:hover{
cursor: pointer;
box-shadow: 3px 3px darkgray;
}
</style>
</head>
<body>
<div id="bigk">
<div id="sck">
<input type="text" value="" style=" 312px;height: 30px;border: none;background-color:gainsboro ;"/>
</div>
<div class="hengk">
<div class="xiaok">7</div>
<div class="xiaok">8</div>
<div class="xiaok">9</div>
<div class="xiaok">/</div>
</div>
<div class="hengk">
<div class="xiaok">4</div>
<div class="xiaok">5</div>
<div class="xiaok">6</div>
<div class="xiaok">*</div>
</div>
<div class="hengk">
<div class="xiaok">1</div>
<div class="xiaok">2</div>
<div class="xiaok">3</div>
<div class="xiaok">-</div>
</div>
<div class="hengk">
<div class="xiaok">0</div>
<div class="xiaok">.</div>
<div class="xiaok">=</div>
<div class="xiaok">+</div>
</div>
</div>
</body>
</html>
<script type="text/javascript">
var num = document.getElementsByClassName("xiaok");
var a = 0;
var b = 0;
var c = 0;
var biao = 0
for(var i = 0 ;i<num.length;i++){
num[i].onclick=function(){
biao++;
document.getElementsByTagName("input")[0].value+=this.innerHTML;
if(biao ==1){
a =Number(this.innerHTML) ;
}else if(biao==2){
b= this.innerHTML;
}else if(biao==3){
c = Number(this.innerHTML);
}
if(this.innerHTML=="="){
switch (b) {
case"+":document.getElementsByTagName("input")[0].value=a+c;break;
case"-":document.getElementsByTagName("input")[0].value=a-c;break;
case"*":document.getElementsByTagName("input")[0].value=a*c;break;
case"/":document.getElementsByTagName("input")[0].value=a/c;break;
}
}
}
}
</script>