DataTransfer: effectAllowed property

The DataTransfer.effectAllowed property specifies the effect that is allowed for a drag operation. The copy operation is used to indicate that the data being dragged will be copied from its present location to the drop location. The move operation is used to indicate that the data being dragged will be moved, and the link operation is used to indicate that some form of relationship or connection will be created between the source and drop locations.

This property should be set in the dragstart event to set the desired drag effect for the drag source. Within the dragenter and dragover event handlers, this property will be set to whatever value was assigned during the dragstart event, thus effectAllowed may be used to determine which effect is permitted.

Assigning a value to effectAllowed in events other than dragstart has no effect.

Value

A string representing the drag operation that is allowed. The possible values are:

none

The item may not be dropped.

copy

A copy of the source item may be made at the new location.

A copy or link operation is permitted.

copyMove

A copy or move operation is permitted.

A link may be established to the source at the new location.

linkMove

A link or move operation is permitted.

move

An item may be moved to a new location.

all

All operations are permitted.

uninitialized

The default value when the effect has not been set, equivalent to all.

Assigning any other value to effectAllowed has no effect and the old value is retained.

Examples

Setting effectAllowed

In this example we set effectAllowed to "move" in the dragstart handler.

HTML

html

<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>

CSS

css

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

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

#target {
  border: 1px solid black;
}

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

JavaScript

js

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());

Result

Specifications

Specification
HTML Standard
# dom-datatransfer-effectallowed-dev

Browser compatibility

BCD tables only load in the browser

See also