<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<meta http-equiv="X-UA-Compatible" content="IE=edge">
<meta name="viewport" content="width=device-width, initial-scale=1.0">
<title>Document</title>
</head>
<body>
<script>
var tuoi = prompt("Nhap tuoi");
// cach 1
var thuong_keo = (tuoi <18) ? "Keo bong":"Khong thuong";
alert (thuong_keo);
// cach 2:
if(tuoi<18)
thuong_keo = "Keo bong";
else
thuong_keo = "Khong thuong";
alert(thuong_keo);
</script>
</body>
</html>