Set operations for objects (as opposed to iterables)

object.js

/**
 * Returns the difference of the first and other objects provided.
 * @param {any} x
 * @param {any} ys
 */
export const difference = (x, ...ys) =>
  Object.fromEntries(
    Object.entries(x).filter(([k, v]) => !ys.some(y => y[k] === v)),
  );

/**
 * Returns the symmetric difference of the two objects provided,
 * using values from `a` if the same key appears in both objects with
 * different values.
 * @param {any} a
 * @param {any} b
 */
export const symmetricDifference = (a, b) => {
  const _a = Object.assign({}, a);
  return Object.fromEntries([
    ...Object.entries(b || {}).filter(
      ([k, v]) => !(_a[k] === v && delete _a[k]),
    ),
    ...Object.entries(_a).filter(([k, v]) => b[k] !== v),
  ]);
};

object.test.js

import { difference, symmetricDifference } from './object';

describe('utils | object | difference', () => {
  it('finds the difference of two objects', () => {
    const a = { a: 5, b: 8, c: 13, d: 21 };
    const b = { b: 8, c: 4, d: 21, e: 10.5 };

    // from a, only 'a' and 'c' do not have the same value in b.
    const expectation = { a: 5, c: 13 };
    expect(difference(a, b)).toEqual(expectation);
  });

  it('finds the difference of three objects', () => {
    const a = { a: 5, b: 8, c: 13, d: 21 };
    const b = { b: 8, c: 4 };
    const c = { c: 4, d: 21, e: 10.5 };

    // from a, only 'a' and 'c' do not have the same value in b.
    const expectation = { a: 5, c: 13 };
    expect(difference(a, b, c)).toEqual(expectation);
  });

  it('throws as expected', () => {
    expect(() => difference({}, null)).not.toThrowError();
    expect(() => difference({}, undefined)).not.toThrow();
    expect(() => difference(null, {})).toThrow();
    expect(() => difference(null, null)).toThrow();
    expect(() => difference(null, undefined)).toThrow();
  });
});

describe('utils | object | symmetricdifference', () => {
  it('finds the difference of two objects', () => {
    const a = { a: 5, b: 8, c: 13, d: 21 };
    const b = { b: 8, c: 4, d: 21, e: 10.5 };

    // `a['b'] == b['b']` and `a['d'] == b['d']`, prefer a['c']
    const expectation = { a: 5, c: 13, e: 10.5 };
    expect(symmetricDifference(a, b)).toEqual(expectation);
  });
});