discourse-custom-wizard/assets/javascripts/discourse/components/custom-wizard-field-category.js

import Component from "@ember/component";
import Category from "discourse/models/category";
import { observes } from "discourse-common/utils/decorators";

export default Component.extend({
  categories: [],

  didInsertElement() {
    this._super(...arguments);
    const property = this.field.property || "id";
    const value = this.field.value;

    if (value) {
      this.set(
        "categories",
        [...value].reduce((result, v) => {
          let val =
            property === "id" ? Category.findById(v) : Category.findBySlug(v);
          if (val) {
            result.push(val);
          }
          return result;
        }, [])
      );
    }
  },

  @observes("categories")
  setValue() {
    const categories = (this.categories || []).filter((c) => !!c);
    const property = this.field.property || "id";

    if (categories.length) {
      this.set(
        "field.value",
        categories.reduce((result, c) => {
          if (c && c[property]) {
            result.push(c[property]);
          }
          return result;
        }, [])
      );
    }
  },
});