<!DOCTYPE html>
<html>
<head lang="en">
<meta charset="UTF-8">
<title>嵌套练习</title>
</head>
<body>
<script>
for (var a=1;a<=9;a++){
for (var b=1;b<=a;b++){
document.write("*");
}
document.write("<br />");
}
</script>
</body>
</html>