Deprecated: This feature is no longer recommended. Though some browsers might still support it, it may have already been removed from the relevant web standards, may be in the process of being dropped, or may only be kept for compatibility purposes. Avoid using it, and update existing code if possible; see the compatibility table at the bottom of this page to guide your decision. Be aware that this feature may cease to work at any time.

Non-standard: This feature is non-standard and is not on a standards track. Do not use it on production sites facing the Web: it will not work for every user. There may also be large incompatibilities between implementations and the behavior may change in the future.

The DataTransfer.mozSetDataAt() method is used to add data to a specific index in the drag event's data transfer object.

A data transfer may store multiple items, each at a given zero-based index. mozSetDataAt() may only be called with an index argument less than mozItemCount in which case an existing item is modified, or equal to mozItemCount in which case a new item is added, and the mozItemCount is incremented by one.

Data should be added in order of preference, with the most specific format added first and the least specific format added last. If data of the given format already exists, it is replaced in the same position as the old data.

The data should be either a string, a boolean value or number type (which will be converted into a string) or an nsISupports.

Note: This method is Firefox-specific.


mozSetDataAt(data, index)
mozSetDataAt(type, data, index)



A string representing the type of the drag data to add to the drag data object.


A nsIVariant representing the data to add to the drag data object.


A unsigned long representing the index of the data to add.

Return value

None (undefined).


This example shows the use of the mozSetDataAt() method in a dragstart handler.

function dragstart_handler(event)
  const dt = event.dataTransfer;
  const idx = dt.mozItemCount;
  // Add two new items to the drag transfer
  if (idx >= 0) {
    dt.mozSetDataAt("text/uri-list","", idx);
    dt.mozSetDataAt("text/html", "Hello World", idx+1);


This method is not defined in any Web standard.

Browser compatibility

BCD tables only load in the browser

See also