Can't get tilemaps widthInPixels value?

Phaser 3
1 
class Map extends Phaser.Tilemaps.Tilemap {
    constructor (scene, mapData)
    {
        super(scene, mapData);        

        this._widthInPixels = this.widthInPixels;
        this._heightInPixels = this.heightInPixels
        console.log(this._widthInPixels); //THIS IS WHERE THE PROBLEM IS :: RETURNS undefined
        scene.physics.world.bounds.width = this._widthInPixels;
        scene.physics.world.bounds.height = this._heightInPixels;        
    }

    get widthInPixels()
    {
        return this._widthInPixels;
    }

    get heightInPixels()
    {
        return this._heightInPixels;
    }
}

Everything works in the above code except I can’t get widthInPixels. The map renders perfectly and physics work and everything is great–except, I can’t get the value of widthInPixels so I can apply world bounds to the map bounds.

I’m not sure why, it’s not working–I’ve checked the documentation, and ‘widthInPixels’ IS in the Tilemap class that I am extending. I’ve provided the mapdata which it is obviously received and working well with since it’s rendering the map. I just don’t understand why this isn’t working. What am I missing?