mongodb & php: how can I store and callback a custom js function to parse data?

Question

Before to submit the doubt that I have, let me start from preamble that I'm definitively a newbie on MongoDB world ;( but I'm going to learn quickly!

The goal that I have is to implement a php class (through the MongoDB/Driver object) to handle the db connection, insert and truncate.

This is the class prepared (today):

class mongodb {
    private $mongo;
    /*
     * Constructor to instantiat db connection
     */
    function __construct($host) {
        $this->db = new MongoDB\Driver\Manager("mongodb://" . $host . "/");
    }
    /*
     * Delete collection (upon $filter parameter);
     */
    function delete($collection, $filter) {
        $bulk = new MongoDB\Driver\BulkWrite;
        $bulk->delete($filter);
        $result = $this->db->executeBulkWrite($collection, $bulk);
    }
    /*
     * Insert dataset
     */
    function insert($collection, $dataset) {
        $bulk = new MongoDB\Driver\BulkWrite;
        if (!is_array($dataset)) $dataset = (array) $dataset;
        foreach($dataset as $k => $ds) {
            $i = $bulk->insert($ds);
        }
        $result = $this->db->executeBulkWrite($collection, $bulk);
    }
    /*
     * Insert js fx into db.system.js collection
     */
    function insertfx($dataset) {
        $bulk = new MongoDB\Driver\BulkWrite;
        $i = $bulk->insert($dataset);
        $result = $this->db->executeBulkWrite("db.system.js", $bulk);
    }
}

If I run this code I don't have any error...

$mongodb = new MongoDb("localhost:27017");
$dataset = [
    [
        '_id' => 23120378,
        'x_origin' => -80,
        'y_origin' => 101,
        'hard_bin_map' => 'FFFFAAAAFFFF|FFAA0101AAFF'
    ],[
        '_id' => 23120379,
        'x_origin' => -80,
        'y_origin' => 101,
        'hard_bin_map' => 'FFFFAAAAFFFF|FFAA0203AAFF'
    ]
];
/*
 * truncate & fill collection st.map
 */
$mongodb->delete("st.map", []);
$mongodb->insert("st.map", $dataset);
/*
 * clean + insert a js function into db.system.js
 */
$mongodb->delete("db.system.js", []);
$mongodb->insertfx(array(
    '_id' => "map2matrix",
    'value' => "function (inputmap, startx, starty, splitsize) {
        inputmap = inputmap.split(',');
        let y = starty;
        matrix = []
        inputmap.forEach((el) => {
            let columns = Math.ceil(el.length / splitsize);
            matrix[y] = [];
            x = startx;
            for (i = 0, o = 0; i < columns; ++i, o += splitsize) {
                matrix[y][x] = el.substr(o, splitsize);
                x++;
            }
            y++;
        });
        return(matrix);
    }"
));

but: a) I can query st.map collection (through MongoDB Compass) b) I cannot see db.system.js containing 'map2matrix'

The goal, at the end (and I would appreciate if someone can support me on how to reach this goal) is to trigger fx map2matrix() passing as input

1. st.map.hard_bin_map (ie: FFFFAAAAFFFF|FFAA0203AAFF)
2. st.map.x_origin (ie: 101)
3. st.map.y_origin (ie: -80)
4. 2 (hardcoded split length)

The expected output has to be a matrix like this

    |   -80     -79     -78     -77     -76     -75
----+-----------------------------------------------
101 |   'FF'    'FF'    'AA'    'AA'    'FF'    'FF'
102 |   'FF'    'AA'    '01'    '01'    'AA'    'FF'

Is it correct that way that I use to store data and function? How can I call fx map2matrix passing values stored on st.map collection?