const toggleTodo = (todo) => {
  return {
    ...todo, // ES7
    completed: !todo.completed,
  };

  /* ES6
    return Object.assign({}, todo, {
    completed: !todo.completed,
  });

  */
};

const testToggleTodo = () => {
  const todoBefore = {
    id: 0,
    test: 'Learn Redux',
    completed: false
  };

  const todoAfter = {
    id: 0,
    test: 'Learn Redux',
    completed: true
  }

  deepFreeze(todoBefore);

  expect(
    toggleTodo(todoBefore)
  ).toEqual(todoAfter);

};

testToggleTodo();
console.log('all tests passed');