If a value is the same as any previously-seen value, undefined is emitted instead.
This version uses a function to create a string key of the object, by default JSON.stringify.
Thus we don't need to store all previously seen objects, just their keys.
Alternatively, if a key function doesn't make sense for the value, use
seenToUndefined, which stores the values (less efficient).
If a value is the same as any previously-seen value, undefined is emitted instead. This version uses a function to create a string key of the object, by default JSON.stringify. Thus we don't need to store all previously seen objects, just their keys.
Alternatively, if a key function doesn't make sense for the value, use seenToUndefined, which stores the values (less efficient).