Intent to Deprecate and Remove: “which” field from KeyboardEvent.idl and MouseEvent.idl
Mustaq Ahmed
Primary eng email
Summary
Remove the “which” field from KeyboardEvent.idl and MouseEvent.idl. Currently the field is already defined in UIEvent.idl. After the proposed removal, KeyboardEvent and MouseEvent instances will simply inherit the field from UIEvent, and the actual field value will remain unchanged for them.
Motivation
Until very recently, only the KeyboardEvent spec had defined the “which” field, and none of the KeyboardEvent & MouseEvent spec did. However, MouseEvent.which was available in both Chrome and Edge. The spec discussion to resolve this discrepancy (https://github.com/w3c/uievents/issues/35) concluded last year with the decision to move the “which” field from KeyboardEvent to UIEvent.
The spec has been fixed this week (https://github.com/w3c/uievents/commit/cfe977225970f4ae2cd2e6430977001b40e9382e). It’s time to align Chrome with the spec.
Interoperability and Compatibility Risk
There is very little compatibility risk because instances of both KeyboardEvent and MouseEvent will have unchanged values for “which”. The only visible change will be the field being inherited from the UIEvent instead, which will change the values of the expressions
KeyboardEvent.prototype.hasOwnProperty(“which”)
and
MouseEvent.prototype.hasOwnProperty(“which”)
from “true” to “false”. Only a handful of websites (if any) should be affected by this because of differences in current implementations, see below.
Interop risk is nearly zero because no two browsers are currently consistent about where they define the “which” field:
https://github.com/w3c/uievents/issues/35#issuecomment-297822323
Edge: Supported, already committed to the change (https://github.com/w3c/uievents/issues/35#issuecomment-148487639).
Firefox: Current implementation already follows the new spec.
Safari: No signal. Currently doesn’t have “which” in MouseEvent.idl.
Alternative implementation suggestion for web developers
The “which” field will still be available in KeyboardEvent and MouseEvent instances through inheritance.
Usage information from UseCounter
KeyboardEvent.which: no data
MouseEvent.which: https://www.chromestatus.com/metrics/feature/timeline/popularity/595
UIEvent.which: https://www.chromestatus.com/metrics/feature/timeline/popularity/598
OWP launch tracking bug
Entry on the feature dashboard
None. This change doesn’t need outreach to developers because the visible change is very small.
Requesting approval to remove too?
Yes
Rick Byers
Chris Harrelson
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