Undefined value during second execution of methods in Angular app

I have a very interesting problem with my fragment of app code.

CategoriesPageComponent:

export class CategoriesPageComponent implements OnInit {

  private categories: Array<Category> = [];
  public randomizedCategories: Array<Category> = [];

  constructor(private router: Router,
              private categoriesService: CategoriesService){


    this.categoriesService.getAllCategories().subscribe((categories) => {
      this.categories = categories;
      this.randomizedCategories = this.randomizeCategories(this.categories, 7);
    });
  }

  ngOnInit() {
  }

  private randomizeCategories(allCategories, numberOfCategories): Array<Category> {
    const categoryArray = [];
    const categoriesIDsArray = this.getRandomArray(1, allCategories.length - 1, numberOfCategories);

    for (var categoryKey in categoriesIDsArray) {

      const subcategoryID = this.getRandomArray(1, allCategories[categoriesIDsArray[categoryKey]].subcategories.length - 1, 1);

      console.log(subcategoryID);
      const wordID = this.getRandomArray(1, allCategories[categoriesIDsArray[categoryKey]].subcategories[subcategoryID].words.length - 1, 1);

      const word: Word[] = allCategories[categoriesIDsArray[categoryKey]].subcategories[subcategoryID].words[wordID];
      const subcategory: Subcategory[] = allCategories[categoriesIDsArray[categoryKey]].subcategories[subcategoryID];
      subcategory['words'] = word;

      const category: Category = allCategories[categoriesIDsArray[categoryKey]];
      category.subcategories = subcategory;

      categoryArray.push(category);
    }

    return categoryArray;
  }

  getRandomArray(min, max, numOfElements) {
    let A = [];
    while (max >= min) {
      A.push(max--);
    }

    A.sort(function () {
      return .5 - Math.random();
    });
    return (numOfElements === 1) ? A[0] : A.slice(0, numOfElements);
  }

  changeCategories(){
    console.log(this.randomizeCategories(this.categories, 7));
  }
}

When the randomizeCategories() methods are executed in the constructor all run very well, but when I have called this method in changeCategories() method when I clicked the button, in the browser's console is throwing the error:

Picture with error

The 49 line of code is:

  const wordID = this.getRandomArray(1, allCategories[categoriesIDsArray[categoryKey]].subcategories[subcategoryID].words.length - 1, 1);

wordID, subcategoryID variables are undefined.

Do you have any idea to resolve this problem?

Thank you for help in advance.