Diff 2 items and group into (toCreate, toUpdate, toDelete)

diff.js

import { find, filter, propEq, reject } from "ramda";

const findFn = (key, selectedItem, items) =>
  find(item => item[key] === selectedItem[key], items);

const diff = (nextItems, currentItems, key, diffFn) => {
  const toCreate = reject(
    item => item[key] && findFn(key, item, currentItems),
    nextItems
  );
  const toUpdate = filter(item => {
    const currentItem = find(propEq(key, item[key]), currentItems);
    return currentItem && diffFn(item, currentItem);
  }, nextItems);
  const toDelete = reject(item => findFn(key, item, nextItems), currentItems);

  return { toCreate, toUpdate, toDelete };
};

export default diff;

diff.test.js

import { deepEqual } from "assert";
import diff from "./index";

const key = "testId";
const diffFn = (a, b) => a.name !== b.name;

const item1 = { testId: 1, name: "First Item" };
const item2 = { testId: 2, name: "Second Item" };
const item3 = { testId: 3, name: "Third Item" };
const item4 = { testId: 4, name: "Fourth Item" };
const item5 = { testId: 5, name: "Fifth Item" };

describe("utils/diff", () => {
  it("should return the to be created items", () => {
    const newItem = { name: "New Item" };
    const nextItems = [newItem];
    const previousItems = [item1, item3];
    const { toCreate } = diff(nextItems, previousItems, key, diffFn);
    deepEqual(toCreate, [newItem]);
  });

  it("should return the to be upated items", () => {
    const updatedItem = { testId: 2, name: "UpdatedItem" };
    const nextItems = [item1, updatedItem, item3];
    const previousItems = [item1, item2, item3];
    const { toUpdate } = diff(nextItems, previousItems, key, diffFn);
    deepEqual(toUpdate, [updatedItem]);
  });

  it("should return the to be deleted items", () => {
    const nextItems = [item1, item3];
    const previousItems = [item1, item2, item3];
    const { toDelete } = diff(nextItems, previousItems, key, diffFn);
    deepEqual(toDelete, [item2]);
  });

  it("should return all to be created, updated and deleted items", () => {
    const updatedItem1 = { testId: 1, name: "UpdatedItem1" };
    const updatedItem2 = { testId: 2, name: "UpdatedItem2" };
    const nextItems = [updatedItem1, updatedItem2, item3, item5];
    const previousItems = [item1, item2, item3, item4];
    const { toCreate, toUpdate, toDelete } = diff(
      nextItems,
      previousItems,
      key,
      diffFn
    );
    deepEqual(toCreate, [item5]);
    deepEqual(toUpdate, [updatedItem1, updatedItem2]);
    deepEqual(toDelete, [item4]);
  });
});