我具有以下涉及猫鼬的功能：

  let username = userObject.username;
  Purchase.find({
    account: username,fufilled: true 
  })
    .populate("keys")
    .exec(function(err,foundPurchases) {
      if (err) {
        return inStockItems;
      } else {
        if (foundPurchases.length === 0) {
          return inStockItems;
        } else {
          // these lists will be a list of IDs of game detail entries in the database
          let listOfReceivedIds = foundPurchases.keys.map(obj => obj.game);

          for (let i = 0; i < inStockItems.length; i++) {
            if (inStockItems.length <= minimum) {
              return inStockItems;
            }

            let currentProductAnalysing = inStockItems[i];
            if (listOfReceivedIds.includes(currentProductAnalysing._id)) {
              console.log("removing product");
              inStockItems.splice(i,1);
            }
          }
          return inStockItems;
        }
      }
    });

我正在运行以下函数，该函数返回undefined

inStockItems = function_name(inStockItems,userObject,amount);

如何重写函数，以便函数返回inStockItems而不是undefined的值。谢谢。