Element: copy イベント

addEventListener("copy", (event) => {});
oncopy = (event) => {};

<div class="source" contenteditable="true">Copy text from this box.</div>
<div class="target" contenteditable="true">And paste it into this one.</div>

div.source,
div.target {
  border: 1px solid gray;
  margin: 0.5rem;
  padding: 0.5rem;
  height: 1rem;
  background-color: #e9eef1;
}

const source = document.querySelector("div.source");
source.addEventListener("copy", (event) => {
  const selection = document.getSelection();
  event.clipboardData.setData("text/plain", selection.toString().toUpperCase());
  event.preventDefault();
});