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The weekOfYear accessor property of Temporal.PlainDateTime instances returns a positive integer representing the 1-based week index in the yearOfWeek of this date, or undefined if the calendar does not have a well-defined week system. The first week of the year is 1. It is calendar-dependent.
The set accessor of weekOfYear is undefined. You cannot change this property directly. To create a new Temporal.PlainDateTime object with the desired new weekOfYear value, use the add() or subtract() method with the appropriate number of weeks.
For general information and more examples, see Temporal.PlainDate.prototype.weekOfYear.