效果图：
刚开始做时没考虑到清零和退格两个功能，嘻嘻，后来加的整体与传统计算器比有点小瑕疵。
代码：
<!DOCTYPE html>
<html>
<head>
<meta charset="utf-8">
<title>js简单计算器</title>
<style type="text/css">
*{
margin:0px;
padding:0px;
}
input{
margin-top:2px;
margin-left:2px;
width:230px;
height:30px;
text-align:right;
}
button{
margin-top:2px;
margin-left:2px;
width:50px;
height:50px;
}
#container{
margin-left:1px;
border:1px solid #E4E4E4;
background:#BBBBBB;
width:235px;
height:215px;
}
</style>
<script>
function onLoad(){
//加载完毕后光标自动对应到输入框
document.getElementById("input").focus();
}
//读取按钮的值，传给输入框
function inputEvent(e){
//把val的值改为每个事件的innerHTML值
var val=e.innerHTML;
//获取input标签
var xsval=document.getElementById("input");
//标签里的value连接每个事件的innerHTML值
xsval.value+=val;
}
//计算出结果
function inputOper(){
var xsval=document.getElementById("input");
xsval.value=eval(document.getElementById("input").value);
}
//清零
function clearNum(){
var xsval=document.getElementById("input");
xsval.value="";
document.getElementById("input").focus();
}
//退格
function backNum(){
var arr=document.getElementById("input");
arr.value=arr.value.substring(0,arr.value.length-1);
}
</script> </head> <body onload="onLoad()"> <input id="input" type="text"> <div id="container"> <div> <button onclick="inputEvent(this)">1</button> <button onclick="inputEvent(this)">2</button> <button onclick="inputEvent(this)">3</button> <button onclick="inputEvent(this)">+</button> </div> <div> <button onclick="inputEvent(this)">4</button> <button onclick="inputEvent(this)">5</button> <button onclick="inputEvent(this)">6</button> <button onclick="inputEvent(this)">-</button> </div> <div> <button onclick="inputEvent(this)">7</button> <button onclick="inputEvent(this)">8</button> <button onclick="inputEvent(this)">9</button> <button onclick="inputEvent(this)">*</button> </div> <div> <button onclick="inputEvent(this)">0</button> <button onclick="inputEvent(this)">.</button> <button onclick="inputOper(this)">=</button> <button onclick="inputEvent(this)">/</button> </div> </div> <button onclick="clearNum()">清零</button> <button onclick="backNum()">退格</button> </body> </html>
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