Essentially, I want to implement the following:

var categories = [];

var products = // some array of product objects

products.map(function(value) {

   if(categories.indexOf(value.Category === -1)) categories.push(value.Category);

});

As result, categories array contains unique list of product categories.

I feel that there should be a better way to do it, but nothing comes to mind.

If there isn't then probably there is no point to use map() in the first place. I could do as simple as

var categories = [];

var products = // some array of product objects

for (var i = 0; i < products.length; i++) {

   if(categories.indexOf(products[i].Category === -1)) categories.push(products[i].Category);

}

UPDATE for those who insist it's a duplicate of how to make an array unique question. I saw that post, and for my situation I don't think it applies. I don't have an array of values that I need to make unique. I have an array of objects and I need to build an array of unique values. The difference might be subtle - but to get to the use case of that topic I would build a non-unique array and then make it unique. Seems even worse than my original solution