HTML Code

<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="utf-8">
  <title>copy uppercase</title>
  <style>
    .source, .target {
      border: 1px solid gray;
      margin: 0.5rem;
      padding: 0.5rem;
      min-height: 1.5rem;
      background-color: #e9eef1;
    }
  </style>
</head>
<body>
  <div class="source" contenteditable="true">Copy text from this box.</div>
  <div class="target" contenteditable="true">And paste it into this one.</div>
  <script>
    var source = document.querySelector("div.source");
    source.addEventListener("copy", function (event) {
      var selection = document.getSelection();
      event.clipboardData.setData("text/plain", selection.toString().toUpperCase());
      event.preventDefault();
    });
  </script>
</body>
</html>

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Element copy — uppercase selection (Try It)