Passing offer.addMyItem or offer.addTheirItem as argument error

[De-pres]

const manager   = new TradeOfferManager ({
  steam: client,
  community: community,
  language: 'en'
});

function make_trade(partner, sellingit, item){ // {"name":"","amount":0,"price":{"ref":0,"keys":0}}
    const offer = manager.createOffer(partner);

    function test(sellinv){
      var sell_added = 0
      for(var c=0; c<=sellinv.length-1; c++){
        if(sellinv[c].market_name == item.name && sell_added < item.amount && sellinv[c].tradable == true){
          seller_add(sellinv[c])
          sell_added+=1;
        }
      }
      
      offer.send((err, status) => {
        if (err) {
          console.log(err);
        } else {
          console.log(`Sent offer. Status: ${status}.`);
          client.chatMessage(partner, `The offer is ${status}`)
        }
      });
    }

  	var buyer_add = offer.addTheirItem
    manager.getUserInventoryContents(partner, appid, contextid, true, (err, inv) => {})

    var seller_add = offer.addMyItem
    manager.getInventoryContents(appid, contextid, true, (err, inv) =>{test(inv)})
  }

I dont know if i am doing it right or its just tradeoffer manager, when changed seller_add() at test function to offer.addMyItem Works fine

error

C:\Users\HP\Desktop\temp\server\main_bot\node_modules\steam-tradeoffer-manager\lib\classes\TradeOffer.js:167
        return addItem(item, this, this.itemsToGive);
                                        ^

TypeError: Cannot read properties of undefined (reading 'itemsToGive')
    at TradeOffer.addMyItem (C:\Users\HP\Desktop\temp\server\main_bot\node_modules\steam-tradeoffer-manager\lib\classes\TradeOffer.js:167:34)
    at test (C:\Users\HP\Desktop\temp\server\main_bot\bot.js:185:11)
    at C:\Users\HP\Desktop\temp\server\main_bot\bot.js:205:72
    at SteamCommunity.<anonymous> (C:\Users\HP\Desktop\temp\server\main_bot\node_modules\steamcommunity\components\users.js:591:5)
    at Request._callback (C:\Users\HP\Desktop\temp\server\main_bot\node_modules\steamcommunity\components\http.js:67:15)
    at Request.self.callback (C:\Users\HP\Desktop\temp\server\main_bot\node_modules\request\request.js:185:22)
    at Request.emit (node:events:513:28)
    at Request.<anonymous> (C:\Users\HP\Desktop\temp\server\main_bot\node_modules\request\request.js:1154:10)
    at Request.emit (node:events:513:28)
    at Gunzip.<anonymous> (C:\Users\HP\Desktop\temp\server\main_bot\node_modules\request\request.js:1076:12)

[Dr. McKay]

This is a JavaScript quirk. You need to do var buyer_add = offer.addTheirItem.bind(offer)