Problems when receiving websocket data(it's json but result is list of number)

Question

I am a beginner developer developing with Dart. There was a problem while working with websockets.

According to the API help, it's obviously JSON. but data is list of number :(

Why does it work this way?

I'll attach my code and results please help :(

I'm sorry I'm not good at English, I hope you understand

--------------code--------------------------------

import 'dart:convert';

import 'package:web_socket_channel/web_socket_channel.dart';

void main() async {
  /// Create the WebSocket channel
  final channel = WebSocketChannel.connect(
    Uri.parse('wss://api.upbit.com/websocket/v1'),
  );

  channel.sink.add(jsonEncode([{"ticket":"test"},{"type":"ticker","codes":["KRW-BTC"]},{"format":"SIMPLE"}])      
  );

  /// Listen for all incoming data

  channel.stream.listen(
    (data) {
      print(data);
    },
    onError: (error) => print(error),
  );
}

ouput flutter: [123, 34, 116, 121, 34, 58, 34, 116, 105, 99, 107, 101, 114, 34, 44, 34, 99, 100, 34, 58, 34, 75, 82, 87, 45, 66, 84, 67, 34......]

expected {"mk":"KRW-BTC","tms":1523531768829,"td":"2018-04-12","ttm":"11:16:03","ttms":1523531763000,"tp":7691000.0,"tv":0.00996719,"ab":"BID","pcp":7429000.00000000,"c":"RISE","cp":262000.00000000,"sid":1523531768829000,"st":"SNAPSHOT"}