It’s a stream.
See these articles
livePreview, MotionJPEG on RICOH THETA - Acquiring the Data Stream
RICOH THETA SC2 livePreview MotionJPEG Single Frame Extraction
This example is in Dart. The main idea is that normally, the HTTP request will close the HTTP connection. There is likely an option to keep the stream open with the JavaScript library you are using. You can then listen to the stream and get the individual JPEG frames to display.
import 'dart:convert';
import 'dart:io';
void main() async {
Uri apiUrl = Uri.parse('http://192.168.1.1/osc/commands/execute');
var client = HttpClient();
Map<String, String> body = {'name': 'camera.getLivePreview'};
var request = await client.postUrl(apiUrl)
..headers.contentType = ContentType("application", "json", charset: "utf-8")
..write(jsonEncode(body));
var response = await request.close();
response.listen((List<int> data) {
print(data);
});
}
If you are familiar with Python, you can also quickly test and view it with this article.
This code has an example of displaying the preview in react native. Maybe you can use it as a reference.