thing.js

// I believe this is doable thanks to the transform-object-rest-spread Babel plugin.
export function formatConditionalAnimals({ bears, cats }) {
	return {
		animals: {
			...(bears != null && { bears }),
			...(cats != null && { cats }),
		},
	}
}

describe('formatConditionalAnimals', () => {
	it('formats the bears only', () => {
		const testBears = 3
		expect(formatConditionalAnimals({ bears: testBears })).toMatchSnapshot()
	})

	it('formats the bears and cats', () => {
		const testBears = 3
		const testCats = 100
		expect(
			formatConditionalAnimals({
				bears: testBears,
				cats: testCats,
			})
		).toMatchSnapshot()
	})
})

thing.js.snap

// Jest Snapshot v1, https://goo.gl/fbAQLP

exports[`formatConditionalAnimals formats the bears and cats 1`] = `
Object {
  "animals": Object {
    "bears": 3,
    "cats": 100,
  },
}
`;

exports[`formatConditionalAnimals formats the bears only 1`] = `
Object {
  "animals": Object {
    "bears": 3,
  },
}
`;