selectionchange-polyfill

A polyfill that fires selectionchange events for Firefox.

Why

Firefox has a select event for when the selection changes within an input or textarea element, but doesn't yet support the general selectionchange event like other major browsers (see feature request). This polyfill models its behavior after Google Chrome’s.

How to Use

Call selectionchange.start() to enable the polyfill and selectionchange.stop() to disable it. Both functions take an optional DOMDocument argument, defaulting to window.document.

Handlers for the selectionchange event should be registered on the document or its .defaultView (its window).

The event is not cancellable and carries no information about the previous or current selection.

<script src="selectionchange.js"></script>
<script>
selectionchange.start();

document.addEventListener('selectionchange', function (event) {
  var sel = this.getSelection();
  console.log('Selected text:', sel.rangeCount ? sel.getRangeAt(0).toString() : null);
});
</script>

Implementation Notes

This polyfill fires selectionchange immediately after most kinds of selection changes (identified with checkmarks below), but there are some unsupported edge cases.

Ways a user can create a new selection:

• ✅ User presses primary mouse button
• ✅ User double-clicks a word or triple-clicks a paragraph
• ❌ User chooses "Select All" from the context (right-click) menu ¹
• ❌ User chooses "Select All" from the Edit menu in the menu bar ¹
• ✅ User types Ctrl+A (or Cmd+A) to select all
• ✅ User tabs to a different focusable element

Ways a user can modify a selection:

• ✅ User moves mouse while holding down primary mouse button
• ✅ User holds down Shift key (and optionally Alt key) and clicks and/or drags mouse
• ✅ User holds down Shift key (and optionally Alt key) and presses an arrow key
• User scrolls the wheel with the primary mouse button down (a Chrome/Safari feature not in Firefox)

Ways a script can create or modify a selection:

• ❌ Using the Selection API ²
• ❌ Modifying the DOM within the selection, at a boundary, or with any overlap ²
• ✅ Moving focus using .focus() or .blur()

If the document’s selection changes via an unsupported mechanism, the selectionchange event will not necessarily fire soon afterwards, but may still fire eventually, after a subsequent user action.

¹ The reason this polyfill doesn’t currently support "Select All" menu item selection is that it doesn’t trigger a DOM event and is less common. Optional support via polling may be added in the future.

² The reason this polyfill doesn’t currently support selection changes via script is that the author’s initial use case didn’t require it. Polling could be used to detect changes via the Selection API. A DOM MutationObserver could be used to detect selection changes resulting from DOM modifications.