no1.html

<!DOCTYPE html>
<html>
<head>
    <script>
        window.alert("hello");
        document.write("HHHH")
//        undefined
        var num = 5;
        document.write(typeof(num))
        document.write("<br>")
        num = undefined;
        document.write(typeof(num))
        document.write("<br>")
        
//        boolean
        document.write(null === undefined)
        
//        datatype of array
        var arr = ['a', 'b', 'c']
        document.write(typeof(arr))
    </script>
</head>
<body>

<h2>What Can JavaScript Do?</h2>

<p id="demo">JavaScript can change the style of an HTML element.</p>

<button type="button" onclick="document.getElementById('demo').style.fontSize='35px'">Click Me!</button>
    
<button type="button" onclick="document.write('HTML Destoryed!')">Destory the html</button>


</body>
</html> 