Storing Variables Across Play

I am having an issue with player.variables. It doesnt exist. What is the correct way to access 

import * as hz from 'horizon/core';

class Mob extends hz.Component<typeof Mob> {
  static propsDefinition = {
    maxHealth: { type: hz.PropTypes.Number, default: 10 },
    xpReward: { type: hz.PropTypes.Number, default: 50 },
  };

  private health!: number;

  start() {
    this.health = this.props.maxHealth;
  }

  takeDamage(amount: number, player: hz.Player) {
    this.health -= amount;
    console.log(`Mob took ${amount} damage! Health left: ${this.health}`);

    if (this.health <= 0) {
      this.onDefeated(player);
    }
  }

  onDefeated(player: hz.Player) {
    console.log(`Mob defeated! Awarding ${this.props.xpReward} XP to ${player.name.get()}`);

    // :white_heavy_check_mark: Retrieve current XP from player variables
    let currentXP = player.variables.get("playerXP") || 0;
    let newXP = currentXP + this.props.xpReward;

    // :white_heavy_check_mark: Store updated XP in player variables
    player.variables.set("playerXP", newXP);
    console.log(`${player.name.get()} now has ${newXP} XP`);

    // :white_heavy_check_mark: Respawn mob after 5 seconds
    this.async.setTimeout(() => {
      this.health = this.props.maxHealth;
      console.log(`Mob respawned!`);
    }, 5000);
  }
}

hz.Component.register(Mob);

the persistent variable? And if you have any documentation links, I would love to look through those! Thanks!