KeyboardEvent: keyIdentifier property
Non-standard: This feature is not standardized. We do not recommend using non-standard features in production, as they have limited browser support, and may change or be removed. However, they can be a suitable alternative in specific cases where no standard option exists.
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.
The deprecated KeyboardEvent.keyIdentifier read-only property returns a "key identifier" string that can be used to determine what key was pressed. Its non-deprecated replacement is KeyboardEvent.key.
Specifications
Not part of any current specification. This property was part of an old draft of the DOM Level 3 Events Specification, but it was removed in later drafts in favor of KeyboardEvent.key.