循环遍历 json 文件中的嵌套数组对象时遇到问题

Question

I have a json file that looks similar to this:

[
 {
  "January": [
      {
           "Date": "Jan 1st",
           "Event": "New Years"
      },
      {
           "Date": "Jan 17th",
           "Event": "Chinese New Year"
      }
    ],
  "February": [
      {
           "Date": "Feb 14th",
           "Event": "Valentine's Day"
      },
      {
           "Date": "Feb 29th",
           "Event": "Leap Year"
      }
    ]
 }
]

The json file is structured like this but has many more events and months.

I've tried using a nested loop, and nothing is being returned in JSX. I'm able to get results if I console log it, so I'm not sure if maybe I'm using the wrong functions and if I should use two map loops instead of a forEach with a map loop inside. Here's what I've tried so far:

{EventsList.forEach((monthObj) => {
    const monthObjKeys = Object.keys(monthObj);
    monthObjKeys.map((monthKey) => {
        const monthArr = monthObj[monthKey];
        return (
            <div className='Events__ItemList'>
                <h1 className='Events__Month'>{monthObjKeys}</h1>
                <div className='Events__Item'>
                    <p className='Events__Date'>{monthArr.Date}</p>
                    <p className='Events__Name'>{monthArr.Name}</p>
                </div>
            </div>
        );
    });
})}

Any help/suggestions would be much appreciated

Answer 1

The main problem is .forEach() does not return anything so either you can use .map() or .flatMap() .

I would suggest a much simpler way to resolve your issue as the following:

 const eventList = [{"January": [{"Date": "Jan 1st","Event": "New Years"},{"Date": "Jan 17th","Event": "Chinese New Year"}],"February": [{"Date": "Feb 14th","Event": "Valentine's Day"},{"Date": "Feb 29th","Event": "Leap Year"}]}] const events = eventList[0]; const months = Object.keys(events); const result = months.flatMap(key => events[key]); console.log(result);

Thus with the result you can .map() through as the following with your JSX :

{
   result.map(({Date, Event}, index) =>
     <div className='Events__ItemList' key={index}>
         <div className='Events__Item'>
            <p className='Events__Date'>{Date}</p>
            <p className='Events__Name'>{Event}</p>
         </div>
     </div>
   ) 
}

You need to use index attribute on every iterated elements as I suggest above.

Read further about List and Keys in the documentation.

Answer 2

Using a forEach doesn't return an array of React component instances, like using .map would. I might recommend trying this (note:untested) :

{EventsList.flatMap((monthObj) => {
    const monthObjKeys = Object.keys(monthObj);
    return monthObjKeys.map((monthKey) => {
        const monthArr = monthObj[monthKey];
        return (
            <div className='Events__ItemList'>
                <h1 className='Events__Month'>{monthObjKeys}</h1>
                <div className='Events__Item'>
                    <p className='Events__Date'>{monthArr.Date}</p>
                    <p className='Events__Name'>{monthArr.Name}</p>
                </div>
            </div>
        );
    });
})}

So two changes mainly: I used flatMap at the top, since each element of this array is itself returning an array, and I RETURNED the result of the .map() call you had.

If I had used .map at the top instead of .flatMap , it would have been an array of arrays of Components, and I don't think React accepts that -- it wants an array of Components - just 1 layer.