Reducer composition with Arrays

以下是我在上面的示例后的练习代码：

// Individual TODO Reducer
const todoReducer = (state,action) => {
    switch(action.type) {
    case 'ADD_TODO':
        return {
            id: action.id,text: action.text,completed: false
          };
    case 'TOGGLE_TODO':
        if (state.id != action.id) return state;

      // This not working
      /*
      return {
        ...state,completed: !state.completed
      };
      */

      //This works
      var newState = {id: state.id,text: state.text,completed: !state.completed};
      return newState;
    default:
        return state;
  }
};

//TODOS Reducer
const todos = (state = [],action) => {
        switch(action.type) {
        case 'ADD_TODO':
       return [
          ...state,todoReducer(null,action)
       ];
       case 'TOGGLE_TODO':
        return state.map(t => todoReducer(t,action));
      default:
        return state;
    }
};

//Test 1
const testAddTodo = () => {
  const stateBefore = [];

  const action = {
      type: 'ADD_TODO',id: 0,text: 'Learn Redux'
  };

  const stateAfter = [{
     id: 0,text: "Learn Redux",completed: false
  }];

  //Freeze
  deepFreeze(stateBefore);
  deepFreeze(action);

  // Test
  expect(
     todos(stateBefore,action)
  ).toEqual(stateAfter);
};

//Test 2
const testToggleTodo = () => {
  const stateBefore = [{id: 0,completed: false
  },{
    id: 1,text: "Go Shopping",completed: false
  }];

  const action = {
      type: 'TOGGLE_TODO',id: 1
  };

  const stateAfter = [{
     id: 0,completed: true
  }];

  //Freeze
  deepFreeze(stateBefore);
  deepFreeze(action);

  // Expect
  expect(
     todos(stateBefore,action)
  ).toEqual(stateAfter);
};

testAddTodo();
testToggleTodo();
console.log("All tests passed");

问题是,在todoReducer函数中,以下语法不起作用：

return {
        ...state,completed: !state.completed
      };

我使用的是Firefox 44.0版,它在控制台中显示以下错误：

Invalid property id

现在我猜我当前版本的Firefox必须支持Spread操作符.
如果它不是,有没有办法添加一些独立的Polyfill来支持这种语法？

这里也是JSFiddle