The readonly previousPriority property of the TaskPriorityChangeEvent interface returns the priority of the corresponding TaskSignal before it was changed and this prioritychange event was emitted.

This is the value that was set in the TaskPriorityChangeEvent constructor argument options.previous.

The new priority of the task can be read from event.target.priority.

A string, indicating the associated task's priority before it was changed. This will be one of: "user-blocking", "user-visible", "background".

The code below shows the previousPriority being obtained in a handler for a prioritychange event.

// Listen for 'prioritychange' events on the controller's signal.
controller.signal.addEventListener("prioritychange", (event) => {
  const previousPriority = event.previousPriority;
  const newPriority = event.target.priority;
  console.log(
    `The priority changed from ${previousPriority} to ${newPriority}.`,
  );
});

A more complete live example can be found in prioritychange event > Examples.