<!DOCTYPE html>
<html>
<head>
<meta charset="utf-8" />
<title>随便敲敲</title>
</head>
<body>
<form action="" >
<lable>加数1</lable>
<input type="text" name="a" id="a" value="1"/>
<lable>加数1</lable>
<input type="text" name="b" id="b" value="2"/>
<input type="submit" value="求和" onclick="sum()" />
</form>
</body>
<script type="text/javascript">
function sum(){
let a=document.getElementById('a');
let b=document.getElementById('b');
let c;
c=parseInt(a.value)+parseInt( b.value);
window.alert(c);
}
</script>
</html>
<!DOCTYPE html>
<html>
<head>
<meta charset="utf-8" />
<title>随便敲敲</title>
</head>
<body>
<form action="" >
<lable>1+2+3+...+20=</lable>
<br />
<span id="a"></span>
</form>
</body>
<script type="text/javascript">
let sum=0;
for(let i=1;i<=20;i++)
{
sum=sum+i;
}
console.log(sum);
let h=document.querySelector("#a");
h.innerHTML=sum;
</script>
</html>
