JS Bin// source http://jsbin.com/cequnu

Redux 10: Avoid object mutations

const toggleTodo = (todo) => {
  // not allowed
  //todo.completed = !todo.completed;
  //this OK
 // return {
 //   id: todo.id,
 //   text: todo.text,
 //   completed: !todo.completed
 // };
  // note the last assign property wins!
  return Object.assign({}, todo, {
    completed: !todo.completed
  });
  //ES7 object spread:
  //return {
  //...todo,
  //completed: !todo.completed
  //};
};

const testToggleTodo = () => {
  const todoBefore = {
    id:0,
    text: 'Learn Redux',
    completed: false
  };
  const todoAfter = {
    id:0,
    text: 'Learn Redux',
    completed: true    
  };
  
  deepFreeze(todoBefore);
  
  expect(
    toggleTodo(todoBefore)
  ).toEqual(todoAfter);
}

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