DataTransfer: свойство effectAllowed

<div>
  <p id="source" draggable="true">
    Select this element, drag it to the Drop Zone and then release the selection
    to move the element.
  </p>
</div>
<div id="target">Drop Zone</div>
<pre id="output"></pre>
<button id="reset">Reset</button>

div {
  margin: 0em;
  padding: 2em;
}

#source {
  color: blue;
  border: 1px solid black;
}

#target {
  border: 1px solid black;
}

#output {
  height: 100px;
  overflow: scroll;
}

function dragstartHandler(ev) {
  log(`dragstart: effectAllowed = ${ev.dataTransfer.effectAllowed}`);

  // Add this element's id to the drag payload so the drop handler will
  // know which element to add to its tree
  ev.dataTransfer.setData("text", ev.target.id);
  ev.dataTransfer.effectAllowed = "move";
}

function dropHandler(ev) {
  log(`drop: effectAllowed = ${ev.dataTransfer.effectAllowed}`);

  ev.preventDefault();
  // Get the id of the target and add the element to the target's DOM
  const data = ev.dataTransfer.getData("text");
  ev.target.appendChild(document.getElementById(data));
}

function dragoverHandler(ev) {
  log(`dragover: effectAllowed = ${ev.dataTransfer.effectAllowed}`);
  ev.preventDefault();
}

const source = document.querySelector("#source");
const target = document.querySelector("#target");

source.addEventListener("dragstart", dragstartHandler);
target.addEventListener("dragover", dragoverHandler);
target.addEventListener("drop", dropHandler);

function log(message) {
  const output = document.querySelector("#output");
  output.textContent = `${output.textContent}\n${message}`;
  output.scrollTop = output.scrollHeight;
}

const reset = document.querySelector("#reset");
reset.addEventListener("click", () => document.location.reload());