<head>
  <title>Lập trình từ đầu</title>
  <script>
  function myFunction() {
    document.getElementById("demo").innerHTML = "Hello World!";
  }
  </script>
</head>
<body>

<p id="demo">abc</p>
<button type="button" onclick="myFunction()">bấm vào</button>

</body>

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External JavaScript

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