oldValue

The previous value of the flag before this modification occurred.

This will be:

• The string representation of the flag's previous value if the flag existed

• null if the flag didn't exist before (i.e., this is the first time it's being set)

• null if the flag was explicitly set to null previously

This property is useful for:

• Tracking changes and creating audit logs

• Implementing undo functionality

• Conditional logic based on previous states

• Event listeners that need to know what changed