How can i reduce to a more concise form my JS array

There is the following piece of code that I would like to reduce to a more concise form. I think I did not make the best decision at all. Simultaneously getting rid of the bug, which consists in limiting the operation of the program from arrays prescribed in advance in the code. Please tell me how it can be done. Jquery library 3.5.1

function FaendKey(arr){
    var Key = [];
    for (zz = 0; zz < arr.length; zz++) {
        if (fileSystem[zz].name == arr[0]) {
            Key.push(zz);
            break;
        }
    }
    if (arr.length > 2) {
        var arrKeyOld = fileSystem[Key[0]].items;
        for (z = 1; z < arr.length-1; z++) {
            var tickKey = FaendKeyOld(arr[z],arrKeyOld);
            Key.push(tickKey);
            arrKeyOld = arrKeyOld[tickKey].items;
        }
    }
    return Key;
}
function onSuccess(data){
    for (k = 0; k < data.d.results.length; k++) {
        var UrlFull = data.d.results[k].ServerRelativeUrl.split('/');
        UrlFull.splice(0,4);
        var idKey = [];
        var idKey = FaendKey(UrlFull);
        if (idKey.length == 1) {
            fileSystem[idKey[0]].items.push(
                {
                    name: data.d.results[k].Name,
                    isDirectory: true, __KEY__: data.d.results[k].UniqueId,
                    dateModified: data.d.results[k].TimeLastModified,
                    items: [],
                }
            )
        } else {
            if (idKey.length == 2) {
                fileSystem[idKey[0]].items[idKey[1]].items.push(
                    {
                        name: data.d.results[k].Name,
                        isDirectory: true, __KEY__: data.d.results[k].UniqueId,
                        dateModified: data.d.results[k].TimeLastModified,
                        items: [],
                    }
                )
            }
            if (idKey.length == 3) {
                fileSystem[idKey[0]].items[idKey[1]].items[idKey[2]].items.push(
                    {
                        name: data.d.results[k].Name,
                        isDirectory: true, __KEY__: data.d.results[k].UniqueId,
                        dateModified: data.d.results[k].TimeLastModified,
                        items: [],
                    }
                )
            }
.. and then a similar structure up to the value idKey.length == 10...

Answer

you can use a recursive function for that

something like this

const saveItem = (keys, data, store) => {
 if(keys.length === 1){
   store.items.push(data)
   return;
  }
  const [key, ...rest] = keys
  return saveItem(rest, data, store[key])
}

than you can call it like this

function onSuccess(data){
    for (k = 0; k < data.d.results.length; k++) {
        var UrlFull = data.d.results[k].ServerRelativeUrl.split('/');
        UrlFull.splice(0,4);
        var idKey = [];
        var idKey = FaendKey(UrlFull);
        const data =  {
                    name: data.d.results[k].Name,
                    isDirectory: true, __KEY__: data.d.results[k].UniqueId,
                    dateModified: data.d.results[k].TimeLastModified,
                    items: [],
                }
       saveItem(idKey, data, fileSystem) 
      }
}