I am building a Node JS webapp that recieves user's facial images, paints AR objects on the image, and send the edited image back to the user. Currently, I'm looking into Spark AR since it seems to be performing very well in Facebook apps like Instagram. But I can't seem to find an API documentation on calling the Spark API within JS or Node.
It is not possible to use Spark for this because Spark does not currently offer an API for third parties to use their engine. Spark is owned by Facebook and only available for use in Facebook products for the time being (Instagram, Messenger, etc).
As an alternative, I recommend looking into this JS face-tracking library called CLM Tracker: https://github.com/auduno/clmtrackr