java_script(3).html

<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <meta http-equiv="X-UA-Compatible" content="ie=edge">
    <title>Document</title>
    <link rel="stylesheet" href="https://maxcdn.bootstrapcdn.com/bootstrap/4.4.1/css/bootstrap.min.css">
  <script src="https://ajax.googleapis.com/ajax/libs/jquery/3.4.1/jquery.min.js"></script>
  <script src="https://cdnjs.cloudflare.com/ajax/libs/popper.js/1.16.0/umd/popper.min.js"></script>
  <script src="https://maxcdn.bootstrapcdn.com/bootstrap/4.4.1/js/bootstrap.min.js"></script>
  <script>
      function check(){
          var i=document.getElementById('a').value;
          if(isNaN(i))
          document.write("you didnot enter a number");
          else
            alert("you entered a number");
      }
  </script>
</head>
<body>
  Enter the value:<input type="text" id="a"><br>  
  <input type="button" value="CHECK" onclick="check();">
</body>
</html>