Javascript map then filter unique array items

Question

I know how to do both things separately, but I'm sure there must be a way to combine them.

I have an array of categories, which I am extracting from an array of objects:

 this.videoCategories = this.videos.map(v => v.category);

But of course there are duplicates in this array. So now I do

this.uniqueVideoCategories = this.videoCategories.filter((item, index) => {
  return this.videoCategories.indexOf(item) === index;
});

Which works fine, I get an array of the categories without dupes. But I'm trying to learn and dry up the code a bit by stringing them together, and this does not work - yields empty array

  constructor(private videoService: VideoService) {
    this.videos = videoService.getVideos();
    this.videoCategories = this.videos
      .map(v => v.category)
      .filter((item, index) => {
        return this.videoCategories.indexOf(item) === index;
      });
    console.log(this.videoCategories);
  }

Answer 1

Array is empty because when you does filtering array return this.videoCategories.indexOf(item) === index;, field this.videoCategories was empty.

Try it:

this.videoCategories = this.videos
    .map(v => v.category)
    .filter((item, index, array) => {
        return array.indexOf(item) === index;
    });