yarn add --dev jest
npm install --dev jest

yarn add --dev @types/jest
npm install --dev @types/jest

    "scripts": {
    	"test": "jest"
    },

test(`test name`, () => {
  expect(1).toBe(1);
});

test('return a user object', () => {
  expect(getUser(1)).toEqual({
    id: 1,
    email: 'user1@test.com'
  });
});

test('number 0 is falsy but string 0 is truthy', () => {
  expect(0).toBeFalsy();
  expect('0').toBeTruthy();
});

test('array', () => {
  const coloers = ['Red', 'Yellow', 'Blue'];
  expect(colors).toHaveLength(3);
  expect(colors).toContain('Yellow');
  expect(colors).not.toContain('Green');
});

test('string', ()=> {
  expect(getUser(1).email).toBe('user1@test.com');
  expect(getUser(2).email).toMatch(/.*test.com$/);
});

function getUser(id) {
  if (id <= 0) throw new Error('Invalid ID');
  return {
    id,
    meail: `user${id}@test.com`
  };
}

test('throw when id is non negative', () => {
  expect(() => getUser(-1).toThrow());
  expect(() => getUser(-1).toThrow('Invalid ID'));
});

it('works with resolves', )_ => {
  expect.assrtions(1);
  return expect(user.getUserName(5)).resolves.toEqual('Paul');
});

it('works with async/awit', async () => {
  expect.assertions(1);
  const data = await user.getUserName(4);
  expect(data).toEqual('Mark');
});

it('works with aysnc/await and resolves', aysnc() => {
  expect.assertions(1);
  await expect(user.getUserName(5)).resolves.toEqual('Paul');
});

// Testing for async errors using Promise.catch.
test('tests error with promises', () => {
  expect.assertions(1);
  return user.getUserName(2).catch(e =>
    expect(e).toEqual({
      error: 'User with 2 not found.',
    }),
  );
});

// Or using async/await.
it('tests error with async/await', async () => {
  expect.assertions(1);
  try {
    await user.getUserName(1);
  } catch (e) {
    expect(e).toEqual({
      error: 'User with 1 not found.',
    });
  }
});