Deduplicate a list by a key function

Merge two pages of API results and you get the same record twice — two different object references holding the same id. A Set is no help, because the objects are not identical references.

Implement uniqueBy(items, keyFn). For each item, call keyFn(item) to get its identity. Return a new array containing only the first item seen for each distinct key, in the order those items appeared in the input. Keys are compared the way a Set compares values, so 1 and "1" are different keys. An empty input returns an empty array. Never modify items.

What it has to do

• Keep the first item for each distinct key and drop later ones.
• Preserve the original relative order of the items you keep.
• Compare keys by value the way Set does, so 1 and "1" are distinct.
• Return [] for an empty input array.
• Do not mutate the input array.

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