16Remove duplicate items from an array

easyAbout 15 minutesCodingarrayssethash-map

Problem

Implement unique(array, getKey). It returns a new array without duplicates, keeping the first occurrence of each item in its original position. The optional getKey(item) decides what counts as "the same", for example an id.

unique([3, 1, 3, 2, 1]); // [3, 1, 2]
unique([NaN, NaN, 0, -0]); // [NaN, 0]
unique(users, (user) => user.id); // first user per id

Requirements

Keep the order of first occurrences.
Treat NaN as equal to NaN, and 0 as equal to -0.
Keep 1 and "1" apart.
Objects are compared by reference unless getKey is given.
Do not change the input. Solve it in O(n) time.

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Step-by-step

[...new Set(array)] removes duplicates and keeps first occurrences, because a Set remembers insertion order. It covers the basic case; the getKey option needs a little more.

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