how to exclude some elements from javascript Array.map() callback

Question

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

i think, you misuse map for simply iterating. the right method is forEach. — Nina Scholz
– Nina Scholz, Commented Dec 25, 2015 at 20:18
@Andy - the value of Bek's answer is pointing out that I can use reduce() to build an array; as opposed to generating a scalar. There were some edits in Bek's answer; so maybe the error was fixed - or maybe I got the idea and implemented it correctly. In any event, it works fine! — Felix
– Felix, Commented Dec 25, 2015 at 20:46

Bek · Accepted Answer · 2015-12-25 20:53:00Z

23

you can use reduce instead of map

var products = [{Category:'vegetable', price: 1}, {Category:'fruits', price: 2}];
var categories = products.reduce(function(sum, product) {
 if(sum.indexOf(product.Category) === -1){
  sum.push(product.Category);
 }
 return sum;
}, []);

edited Dec 25, 2015 at 20:53

answered Dec 25, 2015 at 20:11

Bek

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6 Comments

Felix Over a year ago

reduce will give me a single value (eg., sum). I need array of values (eg., Categories)

Bek Over a year ago

It is not a single value it is array where things getting accumulated

Felix Over a year ago

Wow! Haven't thought of using reduce in this manner! Marking as answer

Nina Scholz Over a year ago

it rewrites product. categories is not defined. the whole object is pushed. indexof does not work on objects.

Bek Over a year ago

my point was to show how reduce could be used not solving specific problem anyways fixed it

|

Nina Scholz · Accepted Answer · 2015-12-25 20:25:53Z

6

Update: A solution with Array.prototype.reduce()

var products = [{ Name: 'milk', price: 2.50, Category: 'groceries' }, { Name: 'shirt', price: 10, Category: 'clothing' }, { Name: 'apples', price: 5, Category: 'groceries' }],
    categories = products.reduce(function (r, a) {
        if (!~r.indexOf(a.Category)) {
            r.push(a.Category);
        }
        return r;
    }, []);

document.write('<pre>' + JSON.stringify(categories, 0, 4) + '</pre>');

edited Dec 25, 2015 at 20:25

answered Dec 25, 2015 at 20:02

Nina Scholz

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6 Comments

Felix Over a year ago

I don't think this will work - within filter categories is always an empty array, and comparison will always return true. Or am I misunderstanding?

Nina Scholz Over a year ago

please supply some example data, the it's more likely to see what you want.

Alexei Levenkov Over a year ago

@Felix this answer shows how to implement what question asking (filter values in functional style vs. imperative), but it is not what you actually need and duplicate Vankat found answers it perfectly.

Felix Over a year ago

Products = [{Name: 'milk', price: 2.50, Category: 'groceries'}, {Name: 'shirt', price: 10, Category: 'clothing'}, {Name: 'apples', price: 5, Category: 'groceries'}] Expected result: Categories = ['groceries', 'clothing']

Andy Over a year ago

@Felix, in future, perhaps don't think of it as a race. Bex might have answered first wrt reduce but the answer was very poor/not checked.

|

Andy · Accepted Answer · 2015-12-25 20:42:31Z

3

map all the values of the object categories out first, then use filter to dispose of the duplicates.

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

var categories = products.map(function (e) {
  return e.category;
}).filter(function (e, i, a) {
  return a.indexOf(e) === i;
}); // [ "A", "B", "D" ]

DEMO

edited Dec 25, 2015 at 20:42

answered Dec 25, 2015 at 20:23

Andy

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1 Comment

Andy Over a year ago

@NinaScholz, does it say somewhere that indexOf was to be avoided?

Community · Accepted Answer · 2017-05-23 12:26:03Z

0

Follow the Below SO Answer:

How to get distinct values from an array of objects in JavaScript?

var flags = [], output = [], l = array.length, i;
for( i=0; i<l; i++) {
    if( flags[array[i].age]) continue;
    flags[array[i].age] = true;
    output.push(array[i].age);
}

edited May 23, 2017 at 12:26

CommunityBot

11 silver badge

answered Dec 25, 2015 at 20:05

Venkat.R

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