ds4/src/module/actor/actor.ts

import { DS4ActorData, DS4ActorDataType } from "./actor-data";

/**
 * Extend the base Actor entity by defining a custom roll data structure which is ideal for the Simple system.
 * @extends {Actor}
 */
export class DS4Actor extends Actor<DS4ActorDataType> {
    /** @override */
    prepareDerivedData(): void {
        const data = this.data;
        this._prepareCombatValues(data);
    }

    _prepareCombatValues(data: ActorData<DS4ActorDataType>): void {
        const hitPointsModifier = getProperty(data, "data.combatValues.hitPoints.modifier") || 0;
        // data.data seems to get initialized with the enitity's data
        const actorData = (data.data as unknown) as DS4ActorData;
        setProperty(
            data,
            "data.combatValues.hitPoints.max",
            actorData.attributes.body.initial + actorData.traits.constitution.initial + 10 + hitPointsModifier,
        );

        const defenseModifier = getProperty(data, "data.combatValues.defense.modifier") || 0;
        setProperty(
            data,
            "data.combatValues.defense.value",
            actorData.attributes.body.initial +
                actorData.traits.constitution.initial +
                this._getArmorValue() +
                defenseModifier,
        );
    }

    _getArmorValue(): number {
        return this.data["items"]
            .filter((item) => ["armor", "shield"].includes(item.type))
            .filter((item) => item.data.equipped)
            .map((item) => item.data.armorValue)
            .reduce((a, b) => a + b, 0);
    }
}