finds duplicate items of self defining properties

checkForItemDuplicates.js

//finds duplicate items of self defining properties, returns [{item1,item1duplicate},{item2,item2duplicate},{...}]
function (e, data){
            const itemList = JSON.parse(data);
            let data = [];
            let currentIndexForData = 0;
            itemList.data.forEach(
                (item,index)=>{
                    let addedToDataThisCycle = false;
                    let addedThisItemToDataThisCycle = false;
                    //current item, will check if any other item (exluding self) is same, add to data if so
                    console.clear();
                    console.log("Loading... "+(index/(itemList.data.length/100.0)).toFixed(2)+'%');
                    itemList.data.forEach(
                        (comparingItem)=>{
                            if(item.inventory_id!==comparingItem.inventory_id){ //exclude self
                                if(isDuplicate(item,comparingItem)){
                                    let itemNotYetUsedAsComparingItem = true;
                                    //check if comparingItem already was an Item
                                    data.data.forEach((dataOfData)=>{
                                        dataOfData.forEach((checkItem)=>{
                                            if(isDuplicate(item,checkItem)){
                                                itemNotYetUsedAsComparingItem=false;
                                            }
                                        });
                                    });
                                    if(itemNotYetUsedAsComparingItem){
                                        //if almost the same item, add this item to a new array in data array(data)
                                        addedToDataThisCycle = true;
                                        if(!addedThisItemToDataThisCycle){
                                            data.data.push([item]);
                                            addedThisItemToDataThisCycle=true;
                                        }
                                        data.data[currentIndexForData].push(comparingItem);
                                    }
                                }
                            }
                        }
                    );
                    //next item in inventory list
                    if(addedToDataThisCycle){
                        //if something has been added to the data, the currentIndex will be changed
                        currentIndexForData++;
                    }
                }
            );
            return data;
        });
// list the properties that will check if it is in fact duplicate.
//in example: 4 properties
function isDuplicate (item,checkItem){
  return item.item.no===checkItem.item.no && item.new_or_used===checkItem.new_or_used && item.color_id===checkItem.color_id && item.description===checkItem.description;
}