uniqueBy

Write uniqueBy(array, key). It returns a new array without duplicates, where two items are duplicates when they have the same key. For each key it keeps the first item, and the items stay in their original order.

key is either a function that takes an item and returns its key, or a property name, in which case an item's key is the value of that property.

Keys are compared the way a Set compares values: NaN matches NaN, 0 matches -0, 1 and "1" are different, and objects match only when they're the same object. array itself never changes.

const users = [
  { id: 1, name: "Ada" },
  { id: 2, name: "Lin" },
  { id: 1, name: "Ada L." },
];
uniqueBy(users, "id");                             // [{ id: 1, name: "Ada" }, { id: 2, name: "Lin" }]
uniqueBy([1.2, 1.8, 2.1], Math.floor);             // [1.2, 2.1]
uniqueBy(["a", "B", "A"], (s) => s.toLowerCase()); // ["a", "B"]

What the tests check

1. 1removes duplicates by a property name

   const users = [{ id: 1, name: "Ada" }, { id: 2, name: "Lin" }, { id: 1, name: "Ada L." }];
   const result = uniqueBy(users, "id");
   expect(result).toEqual([{ id: 1, name: "Ada" }, { id: 2, name: "Lin" }]);
   expect(result[0]).toBe(users[0]);
   

2. 2removes duplicates by a key function

   expect(uniqueBy([1.2, 1.8, 2.1], Math.floor)).toEqual([1.2, 2.1]);
   expect(uniqueBy(["a", "B", "A", "b"], (s) => s.toLowerCase())).toEqual(["a", "B"]);
   

3. 3keeps the first item for each key, in the original order

   const items = [{ id: 2, n: "a" }, { id: 1, n: "b" }, { id: 2, n: "c" }, { id: 1, n: "d" }];
   expect(uniqueBy(items, "id").map((item) => item.n)).toEqual(["a", "b"]);
   

4. 4compares keys the way a Set does

   expect(uniqueBy([NaN, 0, NaN, -0, 1, "1"], (x) => x)).toEqual([NaN, 0, 1, "1"]);
   

5. 5matches object keys only when they're the same object

   const tag = { name: "js" };
   const items = [{ tag }, { tag: { name: "js" } }, { tag }];
   const result = uniqueBy(items, "tag");
   expect(result).toHaveLength(2);
   expect(result[0]).toBe(items[0]);
   expect(result[1]).toBe(items[1]);
   

6. 6treats a missing property as the key undefined

   expect(uniqueBy([{ id: 1 }, {}, { name: "x" }], "id")).toEqual([{ id: 1 }, {}]);
   

7. 7returns a new array and leaves the original alone

   const input = [1, 1, 2];
   const result = uniqueBy(input, (x) => x);
   expect(result).toEqual([1, 2]);
   expect(input).toEqual([1, 1, 2]);
   const distinct = [1, 2, 3];
   expect(uniqueBy(distinct, (x) => x)).not.toBe(distinct);
   

Learn more

• Map & SetLesson
• Equality & samenessLesson
• Write uniqueBy(array, key) to remove duplicates by a computed key, keeping the first of each.Interview question