fvtt-cthulhu-eternal/module/models/skill.mjs

import { SYSTEM } from "../config/system.mjs"
export default class CthulhuEternalSkill extends foundry.abstract.TypeDataModel {
  static defineSchema() {
    const fields = foundry.data.fields
    const schema = {}
    const requiredInteger = { required: true, nullable: false, integer: true }

    schema.description = new fields.HTMLField({ required: true, textSearch: true })
    schema.settings = new fields.StringField({ required: true, initial: "modern", choices: SYSTEM.AVAILABLE_SETTINGS })

    schema.base = new fields.StringField({ required: true, initial: "0" })
    schema.bonus = new fields.NumberField({ ...requiredInteger, required: true, initial: 0, min: 0 })
    schema.diceEvolved = new fields.BooleanField({ required: true, initial: true })
    schema.rollFailed = new fields.BooleanField({ required: true, initial: false })
    schema.isAdversary = new fields.BooleanField({ required: true, initial: false })

    return schema
  }

  /** @override */
  static LOCALIZATION_PREFIXES = ["CTHULHUETERNAL.Skill"]

  prepareDerivedData() {
    super.prepareDerivedData();
    this.skillTotal = this.computeScore();
  }

  computeScore() {
    let actor = this.parent?.actor;
    if ( isFinite(this.base) ) {
      return Number(this.base) + Number(this.bonus);
    }
    if (!actor) {
      return `${this.base} + ${  String(this.bonus)}`;
    }

    // Split the base value per stat : 
    let base = this.base.toLowerCase();
    let char = actor.system.characteristics[base];
    if (!char) {
      ui.notifications.error(`The characteristic ${base} is wrong for actor ${actor.name}`);  
      return `${this.base  } + ${  String(this.bonus)}`;
    }
    let charValue = char.value;
    return charValue + this.bonus
  }
}