注：本人为初学者，欢迎各位大神指教
在线试一试
简单计算器实现效果：
这里写图片描述
HTML代码counter.html

<!DOCTYPE html>
<html>

	<head>
		<meta charset="UTF-8">
		<title>计算器</title>
		<link rel="stylesheet" type="text/css" href="css/counter.css" />
		<script type="text/javascript" src="js/counter.js"></script>
	</head>

	<body>
		<div class="counter">
			<div class="counter_top" >
				<form>
					<input type="text" id="top" value="" />
				</form>
			</div>
			<div class="counter_down">

				<form>
					<input type="button" class="CE" value="清屏" onclick="cl(this.value)" />
					<input type="button" class="CE" value="退格" onclick="cl(this.value)"/>
					<div class="down_right" id="cou">
						<input type="button" class="cou" value="÷" onclick="eva(this.value)"/>
						<input type="button" class="cou" value="×" onclick="eva(this.value)"/>
						<input type="button" class="cou" value="-" onclick="eva(this.value)"/>
						<input type="button" class="cou" value="+" onclick="eva(this.value)"/>
						<input type="button" class="cou" value="=" onclick="eva(this.value)"/>
					</div>
					<div id="down_num" >
						<input type="button" class="num" value="7" onclick="show(this.value)"/>
						<input type="button" class="num" value="8" onclick="show(this.value)"/>
						<input type="button" class="num" value="9" onclick="show(this.value)"/>
						<input type="button" class="num" value="4" onclick="show(this.value)" />
						<input type="button" class="num" value="5" onclick="show(this.value)" />
						<input type="button" class="num" value="6" onclick="show(this.value)"/>
						<input type="button" class="num" value="1" onclick="show(this.value)"/>
						<input type="button" class="num" value="2" onclick="show(this.value)"/>
						<input type="button" class="num" value="3" onclick="show(this.value)"/>
						<input type="button" class="num" value="0" onclick="show(this.value)"/>
						<input type="button" class="num" value="." onclick="show(this.value)"/>
						<input type="button" class="num" value="+/-" onclick="eva(this.value)"/>
					</div>

				</form>

			</div>
		</div>
	</body>

</html>

CSS代码counter.css

.counter{
	background-color: #E0E0E0;
	width: 440px;
	height: 460px;
	padding: 20px;
}

.counter .counter_top{
	width: 440px;
	height: 53px;
	background-color: #fff;
}

.counter .counter_top input{
	width: 440px;
	height: 53px;
	border: 0px;
}

.counter_down .CE{
	width: 152.5px;
	height: 45px;
	margin: 15px 15px 0px 0px;
}

.counter_down .cou{
	width: 95px;
	height: 45px;
	margin-bottom: 15px;
}

.down_right{
	width: 95px;
	float: right;
	margin-top: 15px;
}

.counter_down .num{
	width: 95px;
	height: 45px;
	margin: 15px 16px 0px 0px;
}

js代码counter.js


var a=0;
var b;
var c = 0;

function show(value) {	
	document.getElementById("top").value +=value;		
}

function eva(value){
	c=parseFloat(document.getElementById("top").value);
	
	if (value=="=") {
		if(b=="+"){
			document.getElementById("top").value=(a+c);
		}else if(b=="-"){
			document.getElementById("top").value=(a-c);
		}else if(b=="×"){
			document.getElementById("top").value=(a*c);
		}else if(b=="÷"){
			if(c==0){
				document.getElementById("top").value="分母不能为0";
			}else{
				document.getElementById("top").value=(a/c);
			}	
		}
	}else{
		a=parseFloat(document.getElementById("top").value);
		document.getElementById("top").value="";
		b=value;
		if(b=="+/-"){
			a=-a;
			document.getElementById("top").value=a;
		}
	}	
}

function cl(value){
	if(value=="清屏"){
		document.getElementById("top").value="";
	}else{		document.getElementById("top").value=document.getElementById("top").value.substring(0,(document.getElementById("top").value.length-1))
	}	
}
# html # css
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