text: import { Pipe } from '@angular/core';
  
  @Pipe({
    name: 'categoryList'
  })
  export class CategoryListPipe {
    transform(mediaItems) {
      var categories = [];
      mediaItems.forEach(mediaItem => {
        if (categories.indexOf(mediaItem.category) <= -1) {
          categories.push(mediaItem.category);
        }
      });
      return categories.join(', ');
    }
  }