Variable in IF-Statement doesn't get declared

Question

At the moment iam developing a Game in JavaScript. I have the following Code which runs when an Object ($gameMap.events()[eventId]) is created:

var _GameEvent_initialize = Game_Event.prototype.initialize;
Game_Event.prototype.initialize = function(mapId, eventId) {
    _GameEvent_initialize.call(this, mapId, eventId);
    if (this.isEnemy()) {
            this._eventEnemyId = this.isEnemy();
            console.log(this._eventEnemyId);

            this._eventEnemyHP = $dataEnemies[this._eventEnemyId].params[0];
            console.log(this._eventEnemyHP);
            this._eventEnemyATK = $dataEnemies[this._eventEnemyId].params[2]; // ATK
            this._eventEnemyDEF = $dataEnemies[this._eventEnemyId].params[3]; // DEF
            this._eventEnemyDEX = $dataEnemies[this._eventEnemyId].params[6]; // DEX
            this._eventEnemyLCK = $dataEnemies[this._eventEnemyId].params[7]; // LCK
            console.log("TRUMP!");
            this._isEnemy = true;
    }
        this.test = "lel";
}

The console tells me, that it runs trough the IF. It looks like this:

1
50
TRUMP!

Also the object contains the variable "test" with the value "lel". But the object doesn't contain _eventEnemyHP etc. So i can call $gameMap.events()[eventId].test; but not $gameMap.events()[eventId]._eventEnemyHP;

What am I doing wrong?