The read-only didTimeout property on the IdleDeadline interface is a Boolean value which indicates whether or not the idle callback is being invoked because the timeout interval specified when Window.requestIdleCallback() was called has expired.
If didTimeout is true, the IdleDeadline object's timeRemaining() method will return approximately 0.
Idle callbacks support the concept of a timeout in order to ensure that whatever task they're meant to perform actually happens, even if the user agent never has enough idle time available. Your callback will typically check the value of didTimeout if it needs to perform an action even if the browser is too busy to grant you the time; you should react by performing the needed task or, ideally, a minimal amount of work that can be done to keep things moving along, then schedule a new callback to try again to get the rest of the work done.