JSFiddle - React, Tailwind, and code Playground

JavaScript

var products = [
  { category: 'A' },
  { category: 'B' },
  { category: 'A' },
  { category: 'D' }
];

//var products = ['A', 'B', 'C', 'A'];

var categories = products.map(function (e) {
  return e.category;
}).filter(function (e, i, a) {
  return a.indexOf(e) === i;
});

console.log(out);