Simplest typescript merge

dumb_merge.ts

const merge = (a: any, b: any): any => {
  if ((typeof b !== "undefined" && typeof b !== "object") || Array.isArray(b))
    return b;
  if ((typeof a !== "undefined" && typeof a !== "object") || Array.isArray(a))
    return a;

  a ||= {};
  b ||= {};
  const keys = [...new Set([...Object.keys(a), ...Object.keys(b)])];
  return keys.reduce(
    (obj, k) => ({
      ...obj,
      [k]: merge(a[k], b[k]),
    }),
    {}
  );
};

test("shallow", () => {
  expect(merge({ a: 1 }, { b: 2 })).toEqual({ a: 1, b: 2 });
});

test("deep", () => {
  expect(merge({ a: { x: 1 } }, { a: { y: 2 } })).toEqual({
    a: { x: 1, y: 2 },
  });
});

test("2nd one wins", () => {
  expect(merge({ a: { x: 1 } }, { a: { x: 2 } })).toEqual({
    a: { x: 2 },
  });
});

test("nothing on right", () => {
  expect(merge({ a: { x: 1 } }, {})).toEqual({
    a: { x: 1 },
  });
});

test("nothing on left", () => {
  expect(merge({}, { a: { x: 1 } })).toEqual({
    a: { x: 1 },
  });
});

test("arrays", () => {
  expect(merge({}, { a: [1, 2, 3] })).toEqual({
    a: [1, 2, 3],
  });
});