it("deepKeys should return correct values for sparsed arrays", () => {
  // fixture setup
  // eslint-disable-next-line no-sparse-arrays
  const mockedData = [1, , 3];

  const expectedState = Object.keys(mockedData).map(k => `[${k}]`);

  const sut = () => deepKeys(mockedData);

  // exercise system
  const actualResult = sut();

  // verify outcome
  expect(actualResult).toEqual(expectedState);
});

- Expected  - 0
+ Received  + 1

  Array [
    "[0]",
+   "[1]",
    "[2]",
  ]Jest