HTML Code

<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="utf-8">
  <title>paste 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 target = document.querySelector("div.target");
    target.addEventListener("paste", function (event) {
      event.preventDefault();
      var paste = (event.clipboardData || window.clipboardData).getData("text");
      paste = paste.toUpperCase();
      var selection = window.getSelection();
      if (!selection.rangeCount) return;
      selection.deleteFromDocument();
      selection.getRangeAt(0).insertNode(document.createTextNode(paste));
      selection.collapseToEnd();
    });
  </script>
</body>
</html>

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Element paste — uppercase (Try It)