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<html>
    <head>
        <title>

        </title>
    </head>
    <body>
        <input type ="button" id="clickme" value="Click Me"/>
    </body>
    <script language ="javascript">

    // lay element co id = "clickme" luu vao bien button
    var button = document.getElementById('clickme');
    // khi click vao element chua trong button thi thuc hien
    // trong ham function thong bao hello word
    button.addEventListener('click',function(){
        alert('Hello world');
    });


    </script>
</html>