Can't read a DataSource via api.

[bsofte]

Summary I have no problem reading the local dataSource. Very simple. But no matter what I do, i can't read the api dataSource.

Screenshots This is the result of the local dataSource. No problem.

On the other hand, THIS is the result of api dataSource. No data displayed. Only a loading bar is shown endlessly.

Version Grid version: Toast UI grid v4.0 Web framework: Django 3.2.8

Additional context Here is my code of the dataSource. The commented lines also are the codes I've tried. All failed.

....
        const dataSource = {
            //contentType: 'application/json',
            api: {
                readData: {
                    url: '/api/readData/,
                    //url: '/api_readData.html,
                    //url: '/api_readData.json,
                    //url: 'http://10.20.30.40/api/readData/',
                    //url: 'http://10.20.30.40/api_readData.html',
                    //url: 'http://10.20.30.40/api_readData.json',
                    method: 'GET',
                    //initParams: { param: 'param' }
                },
            },
            //initialRequest: true,
        }
....

And, here are my api codes. All failed as well. 1st try)

{
    'result': True,
    'data': {
        'contents':  [
            {'SeqNum': 1, 'name': 'XXX', 'job_title': 'manager', 'level': '4', 'start_date': '2019-01-01'},
            {'SeqNum': 2, 'name': 'YYY', 'job_title': 'manager', 'level': '3', 'start_date': '2020-01-01'},
	    ...
        ]
    },
}

2nd try)

json_val = {
    'result': True,
    'data': {
        'contents':  [
            {'SeqNum': 1, 'name': 'XXX', 'job_title': 'manager', 'level': '4', 'start_date': '2019-01-01'},
            {'SeqNum': 2, 'name': 'YYY', 'job_title': 'manager', 'level': '3', 'start_date': '2020-01-01'},
	    ...
        ]
    },
}

3rd try)

[
     {'SeqNum': 1, 'name': 'XXX', 'job_title': 'manager', 'level': '4', 'start_date': '2019-01-01'},
     {'SeqNum': 2, 'name': 'YYY', 'job_title': 'manager', 'level': '3', 'start_date': '2020-01-01'},
	  ...
]

I've tried a lot, I've searched so many references about the Toast UI grid. But I can't seem to make it so far. Any comments would be helpful.