If a value is same as any previously-seen value, undefined is emitted instead.
It stores all previous values and compares against them for each new value.
This would likely be not very efficient compared to seenToUndefinedByKey which uses a one-time computed
key and efficient storage of only the keys (using a Set).
If a value is same as any previously-seen value, undefined is emitted instead. It stores all previous values and compares against them for each new value. This would likely be not very efficient compared to seenToUndefinedByKey which uses a one-time computed key and efficient storage of only the keys (using a Set).