reason

abstract val reason: Any?

Reason

Read-only property indicating the reason the operation was aborted; from MDN:

The reason read-only property returns a JavaScript value that indicates the abort reason. The property is undefined when the signal has not been aborted. It can be set to a specific value when the signal is aborted, using AbortController.abort() or AbortSignal.abort(). If not explicitly set in those methods, it defaults to "AbortError" DOMException.

AbortSignal.reason on MDN