ChangedValue

The ChangedValue type describes a Value that has been changed during a transaction, primarily used so that you can indicate whether the transaction should be rolled back.

[ValueOrUndefined, ValueOrUndefined]

It provides both the the old and new Values in a two-part array. These describe the state of the changed Value in the Store at the start of the transaction, and by the end of the transaction.

Hence, an undefined value for the first item in the array means that the Value was added during the transaction. An undefined value for the second item in the array means that the Value was removed during the transaction. An array with two different Values indicates that it was changed. The two-part array will never contain two items of the same value (including two undefined values), even if, during the transaction, a Value was changed to a different value and then changed back.