function on_metadb_changed(handle_list, fromhook){
    CopyValueFromTag('artist', 'album artist', handle_list);
}


function CopyValueFromTag(source, target, handle_list){
    let sourceTfo = fb.TitleFormat(`%${source}%`);
    let targetTfo = fb.TitleFormat(`%${target}%`);
    handle_list.Convert().filter(handle => sourceTfo.EvalWithMetadb(handle) !== targetTfo.EvalWithMetadb(handle)).forEach(handle => {
        new FbMetadbHandleList(handle).UpdateFileInfoFromJSON(JSON.stringify([{[target]: sourceTfo.EvalWithMetadb(handle)}]));
    });
}

function on_metadb_changed(handle_list, fromhook){
   if (!fromhook) CopyValueFromTag('artist', 'album artist', handle_list);
}

This will put the rating values into the rating field of the playback statistics FB plugin and I'd like to copy those values into my
 custom %rating% field in the file tags. Now I'm doing it manually but i'd like to let FB to do it automatically.

Grazie Davide,
but don't waste too much of your time

Do you know if one of the two rating fields is correctly recognized by other music players or apps?

it seems that it's not possible to get any value from the rating field via the fb.TitleFormat() method, neither as '$meta(rating)', nor as '%rating%' whether playback statistics is installed or not. I don't know if this is because spider monkey panel also has a rating field in its database or because I mapped my rating field wrong or just because I suck at coding 

function on_metadb_changed(handle_list, fromhook){
    if (fromhook) SynchronizeRatingTags(handle_list);
}


function SynchronizeRatingTags(handle_list){
    if (utils.CheckComponent('foo_playcount') && !utils.ReadTextFile(`${fb.FoobarPath}\\LargeFieldsConfig.txt`).includes('fieldSpam=rating')){
        let sourceTfo = fb.TitleFormat('[%rating%]');
        let targetTfo = fb.TitleFormat('$meta(rating)');
        handle_list.Convert().filter(handle => sourceTfo.EvalWithMetadb(handle) !== targetTfo.EvalWithMetadb(handle)).forEach(handle => {
            new FbMetadbHandleList(handle).UpdateFileInfoFromJSON(JSON.stringify([{['rating']: sourceTfo.EvalWithMetadb(handle)}]));
        });
    }
}

function on_metadb_changed(handle_list, fromhook){
    if (fromhook && bHasPlugin && !bFieldCheck) {SynchronizeRatingTags(handle_list);}
}

const bHasPlugin = utils.CheckComponent('foo_playcount'); // Call it once! Not on every function call. All these things should be constant during a foobar instance
const bFieldCheck = utils.ReadTextFile(`${fb.FoobarPath}\\LargeFieldsConfig.txt`).includes('fieldSpam=rating');
const sourceTfo = fb.TitleFormat('[%rating%]');
const targetTfo = fb.TitleFormat('$meta(rating)');

function SynchronizeRatingTags(handle_list){ // This part would be faster if you evalmtdb with the entire list at once, and then update all files
        handle_list.Convert().filter(handle => sourceTfo.EvalWithMetadb(handle) !== targetTfo.EvalWithMetadb(handle)).forEach(handle => {
            new FbMetadbHandleList(handle).UpdateFileInfoFromJSON(JSON.stringify([{['rating']: sourceTfo.EvalWithMetadb(handle)}]));
        });
}

To be honest, I get a weird popup message saying it's not possible to update a doppelganger track in a directory that I don't even have on my pc, but I'm pretty sure it's not coming from the script, probably from something messed-up in my index-data files

let relative_paths = handle_list.GetLibraryRelativePaths();

if (fb.IsMetadbInMediaLibrary(handle)) {
...
}

const sourceTfo = fb.TitleFormat('[%rating%]###%path%');
,,,
let stringArray= tfo.EvalWithMetadbs(handle_list);
let stringArray_length= stringArray.length;
for (let i = 0; i < stringArray_length; i++) {
let [rating, path] = stringArray[i].split('###');
//do your thing with ratings and path here
}

const sourceAndTargetTfo = fb.TitleFormat('[%rating%])###$meta(rating)###%path%');
...
let [rating, ratingtag, path] = stringArray[i].split('###');

const Tfo =  fb.TitleFormat('$ifequal(%rating%,$meta(rating),%path%,)')

let stringArray= tfo.EvalWithMetadbs(handle_list).filter(Boolean)

Looping over a handle list and then creating a NEW handle list for each item is special kinds of stupid. 

There's a reason why UpdateFileInfoFromJSON is implemented as a handle list method only.

var items = ...
var to_tag = fb.GetQueryItems(items, "SOME QUERY GOES HERE");
if (to_tag.Count > 0) {
	var arr = [];
	for (var i = 0; i< to_tag.Count; i++) {
		arr.push({"tag" : "tag_value"});
	}
	to_tag.UpdateFileInfoFromJSON(JSON.stringify(arr));
}

var items = ...
var arr = [];
var to_tag = fb.CreateHandleList();
for (var i = 0; i < items.Count; i++) {
	if (SOME_CONDITION) {
		to_tag.Add(items[i]);
		arr.push({"tag" : "tag_value"});
	}
}
if (to_tag.Count > 0) {
	to_tag.UpdateFileInfoFromJSON(JSON.stringify(arr));
}

If you ever try and tag large amount of items PROPERLY, you'll notice fb2k will spawn a progress dialog because doing it the right way, items are queued and the files are updated sequentially, one at a time. You really do not want to test your code on large amounts of items at once.

Ah... there is also an option in preferences. (Though it would also write the other statistics to tags, so a script might be preferable)

Preferences > Advanced > Tools > Playback Statistics > Auto sync file tags...

Now, if you have a handle list but you don't want to tag every item in it, then you must filter it first. There are 2 ways of doing this...

function CopyValueFromTag(source, target, handle_list){
    let sourceTfo = fb.TitleFormat(`%${source}%`);
    let targetTfo = fb.TitleFormat(`%${target}%`);
    let queryItemsArray = handle_list.Convert().filter(handle => sourceTfo.EvalWithMetadb(handle) !== targetTfo.EvalWithMetadb(handle));
	if (queryItemsArray.length > 0){
		newValuesArray = queryItemsArray.map(handle => {return {[target]: sourceTfo.EvalWithMetadb(handle)}});
		new FbMetadbHandleList(queryItemsArray).UpdateFileInfoFromJSON(JSON.stringify(newValuesArray));
	}
}

let to_tag = fb.GetQueryItems(fb.GetLibraryItems(), "NOT %rating% EQUAL '$meta(rating)'");;
let tag_valueArray =  fb.TitleFormat('%rating%').EvalWithMetadb(to_tag);
if (to_tag.Count > 0) {
	var arr = [];
	for (var i = 0; i< to_tag.Count; i++) {
		arr.push({"rating" : tag_valueArray[i]});
	}
	to_tag.UpdateFileInfoFromJSON(JSON.stringify(arr));
}

const Tfo =  fb.TitleFormat('$ifequal(%rating%,$meta(rating),%rating%,)');
const tag_valueArray = tfo.EvalWithMetadbs(handle_list);
if (tag_valueArray.filter(Boolean).length > 0) {
	var arr = [];
	var i = 0;
	var j = 0;
	while ( i < handle_list.Count) { //size changes while filtering....
		if (tag_valueArray[i]) {
		 arr.push({"rating" : tag_valueArray[j]});
		  i++;
		} else {handle_list.RemoveById(i)}
		 j++; // but  tag_valueArray is constant
	}
	handle_list.UpdateFileInfoFromJSON(JSON.stringify(arr));
}

Is this also bad practice?

//bad
fb.GetLibraryItems().Convert().forEach((item) => {
    if (fb.TitleFormat("%blah%").EvalWithMetadb(item)....
});