Code_doPost.gs

const channelToken = 'your_messaging_api_channel_token';

// `e` is the data provided by Line 
function doPost(e) {
  var value = JSON.parse(e.postData.contents);
  try {
    var events = value.events;
    if (events != null) {
      for (var i in events) {
        var event = events[i];
        var type = event.type;
        var replyToken = event.replyToken; // token to reply message
        var sourceId = LineHelpers.getSourceId(event.source);
        var userId = event.source.userId;
        switch (type) {
          case 'postback':
            break;
          case 'message':
            var messageText = event.message.text; // message send by user
            replyMsg(userId, replyToken, messageText);
            break;
          default:
            break;
        }
      }
    }
  } catch(ex) {
    console.log(ex);
  }
}

var LineHelpers = (function (helpers) {
  'use strict';
  helpers.getSourceId = function (source) {
    try {
      switch (source.type) {
        case 'user':
          return source.userId;
          break;
        case 'group':
          return source.groupId;
          break;
        case 'room':
          return source.roomId;
          break;
        default:
          console.log('LineHelpers, getSourceId, invalid source type!');
          break;
      }
    } catch (ex) {
      console.log('LineHelpers, getSourceId, ex = ' + ex);
    }
  }; 
  return helpers;
})(LineHelpers || {});