In React, the
onFocus event is called when the element receives focus
onBlur event is called when focus has left the element.
There are 4 types of native focus events,
focus/blur which do not
focusin/focusout which bubble.
Before React 17, the
onBlur events were internally mapped to
blur events in the capture phase, giving an impression as if the events bubbled.
In the world of Keyboard accessibility,
relatedTarget plays an important role.
IE 9-11 has native support for relatedTarget in
focusout, but in React’s
onBlur event, its value is null.
This is because React
for the availability of event capturing to use the blur/focus events.
If capturing is not supported, it uses the focusin/focusout event (< IE 9).
Starting IE 9, IE team has added support for addEventListener and trapping capturing events.
This means that IE 8 and before, React will have relatedTarget on onFocus and onBlur handlers but in IE 9-11 it will be null.
To understand this impact, let’s check out the example of navigating a dropdown menu using a keyboard. When we try to navigate to the list using the tab, the menu item should be focussed and when we move out of the menu it the dropdown menu should be closed.
Before React 17, while trying to navigate through the list using the tab in IE 9-11, the dropdown menu gets closed. This is because
relatedTarget is null.
With the changes in React 17,
focusout instead of
blur events thereby making
relatedTarget available in IE 9-11.
These changes align React closer with the browser behavior and improve interoperability.
Due to the breaking changes, React will not support Firefox versions earlier than 52 as these versions do not support focusin and focusout.
Check out the pull request to know more.