Slightly off topic but this code in example #5:

  get() {
    return new Promise((resolve, reject) => {
      fetch(`${baseAPI}/heroes`)
        .then(response => response.json())
        .then(json => resolve(json))
        .catch(err => {
          reject (err);
        });
    });
  }

Why not just:

  get() {
    return fetch(`${baseAPI}/heroes`)
      .then(response => response.json());
  }