JavaScript questions that don't deserve their own thread

Game_Map.prototype.width = function() {
    return $dataMap.width;
};

Game_Map.prototype.height = function() {
    return $dataMap.height;
};

Game_Map.prototype.isValid = function(x, y) {
    return x >= 0 && x < this.width() && y >= 0 && y < this.height();
};

Game_Map.prototype.tileId = function(x, y, z) {
    const width = $dataMap.width;
    const height = $dataMap.height;
    return $dataMap.data[(z * height + y) * width + x] || 0;
};

Game_Map.prototype.regionId = function(x, y) {
    return this.isValid(x, y) ? this.tileId(x, y, 5) : 0;
};

var testID = 1;
PluginManager.registerCommand("DreamDungeon", "MoveTest", args => {
    console.log(args.mapId);
    testID = args.regionID;
    var data = LoadDataFile(args.mapId, loadSuccess)
}); 
function LoadDataFile (mapId, onload, context) {
var src = 'Map%1.json'.format(mapId.padZero(3));
    var xhr = new XMLHttpRequest();
    var url = 'data/' + src;
    var result = null;
    xhr.open('GET', url);
    xhr.overrideMimeType('application/json');
    xhr.onload = function() {
        if (xhr.status < 400) {
            result = JSON.parse(xhr.responseText);
            result.id = mapId;
            // On a succesful map load we then call our 'onload' function and pass in the result.
            onload.call(context, result)
        }
    };
    xhr.onerror = function(err) {
        console.log("Failed to load Map Info");
    };
    xhr.send();
};
// This function will be called when the data file is loaded
var loadSuccess = function (data) {
    console.log('Load Success', data);
    var pos = FindRegionID(testID, data);
    $gamePlayer.reserveTransfer(data.id, pos.x, pos.y, 0, 0);
    
}
function FindRegionID(id, data) {
    for (let i = 0; i < data.width; i++) 
    {
        for (let j = 0; j < data.height; j++) 
        {
            if(data.regionId(i,j) == id) //data.regionID does not exist
            {
                console.log('Region ID:', id, "found at (", i, ",", j, ")");
                //output should be the x,y pos for regionid
                var position = { x: i, y: j };
                return position;
            }
        }
    }
};