一 代码
稍微看看即可，很简单。
<!DOCTYPE html>
<html>
<head>
<meta charset="UTF-8">
<title></title>
<script type="text/javascript">
var n1=10;
var n2=20;
var n = n1++;
console.log('n='+n); // 10
console.log('n1='+n1); // 11
n = ++n1
console.log('n='+n); //12
console.log('n1='+n1); //12
n = n2--;
console.log('n='+n); //20
console.log('n2='+n2); //19
n = --n2;
console.log('n='+n); //18
console.log('n2='+n2); //18
</script>
</head>
<body>
</body>
</html>