Get all unique values in a JavaScript array (remove duplicates)

I have an array of numbers that I need to make sure are unique. I found the code snippet below on the internet and it works great until the array has a zero in it. I found this other script here on SO that looks almost exactly like it, but it doesn't fail.

So for the sake of helping me learn, can someone help me determine where the prototype script is going wrong?

Array.prototype.getUnique = function() {
 var o = {}, a = [], i, e;
 for (i = 0; e = this[i]; i++) {o[e] = 1};
 for (e in o) {a.push (e)};
 return a;

More answers from duplicate question:

Similar question:

With JavaScript 1.6 / ECMAScript 5 you can use the native filter method of an Array in the following way to get an array with unique values:

function onlyUnique(value, index, self) { 
    return self.indexOf(value) === index;

// usage example:
var a = ['a', 1, 'a', 2, '1'];
var unique = a.filter( onlyUnique ); // returns ['a', 1, 2, '1']

The native method filter will loop through the array and leave only those entries that pass the given callback function onlyUnique.

onlyUnique checks, if the given value is the first occurring. If not, it must be a duplicate and will not be copied.

This solution works without any extra library like jQuery or prototype.js.

It works for arrays with mixed value types too.

For old Browsers (<ie9), that do not support the native methods filter and indexOf you can find work arounds in the MDN documentation for filter and indexOf.

If you want to keep the last occurrence of a value, simple replace indexOf by lastIndexOf.

With ES6 it could be shorten to this:

// usage example:
var myArray = ['a', 1, 'a', 2, '1'];
var unique = myArray.filter((v, i, a) => a.indexOf(v) === i); 

// unique is ['a', 1, 2, '1']

Thanks to Camilo Martin for hint in comment.

ES6 has a native object Set to store unique values. To get an array with unique values you could do now this:

var myArray = ['a', 1, 'a', 2, '1'];

let unique = [ Set(myArray)]; 

// unique is ['a', 1, 2, '1']

The constructor of Set takes an iterable object, like Array, and the spread operator ... transform the set back into an Array. Thanks to Lukas Liese for hint in comment.

Updated answer for ES6/ES2015: Using the Set, the single line solution is:

var items = [4,5,4,6,3,4,5,2,23,1,4,4,4]
var uniqueItems = Array.from(new Set(items))

Which returns

[4, 5, 6, 3, 2, 23, 1]

As le_m suggested, this can also be shortened using spread operator , like

var uniqueItems = [ Set(items)]

I realise this question has more than 30 answers already. But I've read through all the existing answers first and made my own research.

I split all answers to 4 possible solutions:

  1. Use new ES6 feature: [ Set( [1, 1, 2] )];
  2. Use object { } to prevent duplicates
  3. Use helper array [ ]
  4. Use filter + indexOf

Here's sample codes found in answers:

Use new ES6 feature: [ Set( [1, 1, 2] )];

function uniqueArray0(array) {
  var result = Array.from(new Set(array));
  return result    

Use object { } to prevent duplicates

function uniqueArray1( ar ) {
  var j = {};

  ar.forEach( function(v) {
    j[v+ '::' + typeof v] = v;

  return Object.keys(j).map(function(v){
    return j[v];

Use helper array [ ]

function uniqueArray2(arr) {
    var a = [];
    for (var i=0, l=arr.length; i<l; i++)
        if (a.indexOf(arr[i]) === -1 && arr[i] !== '')
    return a;

Use filter + indexOf

function uniqueArray3(a) {
  function onlyUnique(value, index, self) { 
      return self.indexOf(value) === index;

  // usage
  var unique = a.filter( onlyUnique ); // returns ['a', 1, 2, '1']

  return unique;

And I wondered which one is faster. I've made sample Google Sheet to test functions. Note: ECMA 6 is not avaliable in Google Sheets, so I can't test it.

Here's the result of tests: enter image description here

I expected to see that code using object { } will win because it uses hash. So I'm glad that tests showed best results for this algorithm in Chrome and IE. Thanks to @rab for the code.

You can also use underscore.js.

console.log(_.uniq([1, 2, 1, 3, 1, 4]));
<script src=""></script>

which will return:

[1, 2, 3, 4]

One Liner, Pure JavaScript

With ES6 syntax

list = list.filter((x, i, a) => a.indexOf(x) == i)

x --> item in array
i --> index of item
a --> array reference, (in this case "list")

enter image description here

With ES5 syntax

list = list.filter(function (x, i, a) { 
    return a.indexOf(x) == i; 

Browser Compatibility: IE9+

I have since found a nice method that uses jQuery

arr = $.grep(arr, function(v, k){
    return $.inArray(v ,arr) === k;

Note: This code was pulled from Paul Irish's duck punching post - I forgot to give credit :P

Shortest solution with ES6: [ Set( [1, 1, 2] )];

Or if you want to modify the Array prototype (like in the original question):

Array.prototype.getUnique = function() {
    return [ Set( [this] )];

EcmaScript 6 is only partially implemented in modern browsers at the moment (Aug. 2015), but Babel has become very popular for transpiling ES6 (and even ES7) back to ES5. That way you can write ES6 code today!

If you're wondering what the ... means, it's called the spread operator. From MDN: «The spread operator allows an expression to be expanded in places where multiple arguments (for function calls) or multiple elements (for array literals) are expected». Because a Set is an iterable (and can only have unique values), the spread operator will expand the Set to fill the array.

Resources for learning ES6:

Simplest solution:

var arr = [1, 3, 4, 1, 2, 1, 3, 3, 4, 1];
console.log([ Set(arr)]);


var arr = [1, 3, 4, 1, 2, 1, 3, 3, 4, 1];
console.log(Array.from(new Set(arr)));

The simplest, and fastest (in Chrome) way of doing this:

Array.prototype.unique = function() {
    var a = [];
    for (var i=0, l=this.length; i<l; i++)
        if (a.indexOf(this[i]) === -1)
    return a;

Simply goes through every item in the array, tests if that item is already in the list, and if it's not, push to the array that gets returned.

According to jsPerf, this function is the fastest of the ones I could find anywhere - feel free to add your own though.

The non-prototype version:

function uniques(arr) {
    var a = [];
    for (var i=0, l=arr.length; i<l; i++)
        if (a.indexOf(arr[i]) === -1 && arr[i] !== '')
    return a;


When also needing to sort the array, the following is the fastest:

Array.prototype.sortUnique = function() {
    var last_i;
    for (var i=0;i<this.length;i++)
        if ((last_i = this.lastIndexOf(this[i])) !== i)
            this.splice(i+1, last_i-i);
    return this;

or non-prototype:

function sortUnique(arr) {
    var last_i;
    for (var i=0;i<arr.length;i++)
        if ((last_i = arr.lastIndexOf(arr[i])) !== i)
            arr.splice(i+1, last_i-i);
    return arr;

This is also faster than the above method in most non-chrome browsers.

PERFORMANCE ONLY! this code is probably 10X faster than all the codes in here *works on all browsers and also has the lowest memory impact.... and more

if you don't need to reuse the old array;btw do the necessary other operations before you convert it to unique here is probably the fastest way to do this, also very short.

var array=[1,2,3,4,5,6,7,8,9,0,1,2,1];

then you can try this

var array = [1, 2, 3, 4, 5, 6, 7, 8, 9, 0, 1, 2, 1];

function toUnique(a, b, c) { //array,placeholder,placeholder
  b = a.length;
  while (c = --b)
    while (c--) a[b] !== a[c] || a.splice(c, 1);
  return a // not needed ;)
//[3, 4, 5, 6, 7, 8, 9, 0, 2, 1]

I came up with this function reading this article...

I don't like the for loop. it has to many parameters.i like the while-- loop. while is the fastest loop in all browsers except the one we all like so much... chrome.

anyway i wrote the first function that uses while.And yep it's a little faster than the function found in the article.but not enough.unique2()

next step use modern js.Object.keys i replaced the other for loop with js1.7's Object.keys... a little faster and shorter (in chrome 2x faster) ;). Not enough!.unique3().

at this point i was thinking about what i really need in MY unique function. i don't need the old array, i want a fast function. so i used 2 while loops + splice.unique4()

Useless to say that i was impressed.

chrome: the usual 150,000 operations per second jumped to 1,800,000 operations per second.

ie: 80,000 op/s vs 3,500,000 op/s

ios: 18,000 op/s vs 170,000 op/s

safari: 80,000 op/s vs 6,000,000 op/s

Proof or better use console.time... microtime... whatever

unique5() is just to show you what happens if you want to keep the old array.

Don't use Array.prototype if yu don't know what your doing. i just did alot of copy and past. Use Object.defineProperty(Array.prototype,...,writable:false,enumerable:false}) if you want to create a native prototype.example:


NOTE: your old array is destroyed/becomestheunique after this operation.

if you can't read the code above ask, read a javascript book or here are some explainations about shorter code.

some are using indexOf ... don't ...

for empty arrays


Many of the answers here may not be useful to beginners. If de-duping an array is difficult, will they really know about the prototype chain, or even jQuery?

In modern browsers, a clean and simple solution is to store data in a Set, which is designed to be a list of unique values.

const cars = ['Volvo', 'Jeep', 'Volvo', 'Lincoln', 'Lincoln', 'Ford'];
const uniqueCars = Array.from(new Set(cars));

The Array.from is useful to convert the Set back to an Array so that you have easy access to all of the awesome methods (features) that arrays have. There are also other ways of doing the same thing. But you may not need Array.from at all, as Sets have plenty of useful features like forEach.

If you need to support old Internet Explorer, and thus cannot use Set, then a simple technique is to copy items over to a new array while checking beforehand if they are already in the new array.

// Create a list of cars, with duplicates.
var cars = ['Volvo', 'Jeep', 'Volvo', 'Lincoln', 'Lincoln', 'Ford'];
// Create a list of unique cars, to put a car in if we haven't already.
var uniqueCars = [];

// Go through each car, one at a time.
cars.forEach(function (car) {
    // The code within the following block runs only if the
    // current car does NOT exist in the uniqueCars list
    // - a.k.a. prevent duplicates
    if (uniqueCars.indexOf(car) === -1) {
        // Since we now know we haven't seen this car before,
        // copy it to the end of the uniqueCars list.

To make this instantly reusable, let's put it in a function.

function deduplicate(data) {
    if (data.length > 0) {
        var result = [];

        data.forEach(function (elem) {
            if (result.indexOf(elem) === -1) {

        return result;

So to get rid of the duplicates, we would now do this.

var uniqueCars = deduplicate(cars);

The deduplicate(cars) part becomes the thing we named result when the function completes.

Just pass it the name of any array you like.

["Defects", "Total", "Days", "City", "Defects"].reduce(function(prev, cur) {
  return (prev.indexOf(cur) < 0) ? prev.concat([cur]) : prev;
 }, []);

[0,1,2,0,3,2,1,5].reduce(function(prev, cur) {
  return (prev.indexOf(cur) < 0) ? prev.concat([cur]) : prev;
 }, []);

We can do this using ES6 sets:

var duplicatedArray = [1, 2, 3, 4, 5, 1, 1, 1, 2, 3, 4];
var uniqueArray = Array.from(new Set(duplicatedArray));


//The output will be

uniqueArray = [1,2,3,4,5];

This prototype getUnique is not totally correct, because if i have a Array like: ["1",1,2,3,4,1,"foo"] it will return ["1","2","3","4"] and "1" is string and 1 is a integer; they are different.

Here is a correct solution:

Array.prototype.unique = function(a){
    return function(){ return this.filter(a) }
}(function(a,b,c){ return c.indexOf(a,b+1) < 0 });


var foo;
foo = ["1",1,2,3,4,1,"foo"];

The above will produce ["1",2,3,4,1,"foo"].


a.filter(e=>!(t[e]=e in t)) 

O(n) performance; we assume your array is in a and t={}. Explanation here (+Jeppe impr.)

let t={}, unique= a=> a.filter(e=>!(t[e]=e in t));

// "stand-alone" version working with global t:
// a1.filter((t={},e=>!(t[e]=e in t)));

// Test data
let a1 = [5,6,0,4,9,2,3,5,0,3,4,1,5,4,9];
let a2 = [[2, 17], [2, 17], [2, 17], [1, 12], [5, 9], [1, 12], [6, 2], [1, 12]];
let a3 = ['Mike', 'Adam','Matt', 'Nancy', 'Adam', 'Jenny', 'Nancy', 'Carl'];

// Results
console.log(JSON.stringify( unique(a1) ))
console.log(JSON.stringify( unique(a2) ))
console.log(JSON.stringify( unique(a3) ))

[ Set(duplicates)]

This is the simplest one and referenced from MDN Web Docs.

const numbers = [2,3,4,4,2,3,3,4,4,5,5,6,6,7,5,32,3,4,5]
console.log([ Set(numbers)]) // [2, 3, 4, 5, 6, 7, 32]

Without extending Array.prototype (it is said to be a bad practice) or using jquery/underscore, you can simply filter the array.

By keeping last occurrence:

    function arrayLastUnique(array) {
        return array.filter(function (a, b, c) {
            // keeps last occurrence
            return c.indexOf(a, b + 1) < 0;

or first occurrence:

    function arrayFirstUnique(array) {
        return array.filter(function (a, b, c) {
            // keeps first occurrence
            return c.indexOf(a) === b;

Well, it's only javascript ECMAScript 5+, which means only IE9+, but it's nice for a development in native HTML/JS (Windows Store App, Firefox OS, Sencha, Phonegap, Titanium, ...).

If you're using Prototype framework there is no need to do 'for' loops, you can use like this:

var a = Array.uniq();  

Which will produce a duplicate array with no duplicates. I came across your question searching a method to count distinct array records so after


I used


and there was my simple result. p.s. Sorry if i misstyped something

edit: if you want to escape undefined records you may want to add


before, like this:

var a = Array.compact().uniq();  

Now using sets you can remove duplicates and convert them back to the array.

var names = ["Mike","Matt","Nancy", "Matt","Adam","Jenny","Nancy","Carl"];

console.log([ Set(names)])

Another solution is to use sort & filter

var names = ["Mike","Matt","Nancy", "Matt","Adam","Jenny","Nancy","Carl"];
var namesSorted = names.sort();
const result = namesSorted.filter((e, i) => namesSorted[i] != namesSorted[i+1]);

That's because 0 is a falsy value in JavaScript.

this[i] will be falsy if the value of the array is 0 or any other falsy value.

Array.prototype.getUnique = function() {
    var o = {}, a = []
    for (var i = 0; i < this.length; i++) o[this[i]] = 1
    for (var e in o) a.push(e)
    return a

I had a slightly different problem where I needed to remove objects with duplicate id properties from an array. this worked.

let objArr = [{
  id: '123'
}, {
  id: '123'
}, {
  id: '456'

objArr = objArr.reduce((acc, cur) => [
  ...acc.filter((obj) => !==, cur
], []);


Es6 based solution...

var arr = [2, 3, 4, 2, 3, 4, 2];
const result = [ Set(arr)];

From Shamasis Bhattacharya's blog (O(2n) time complexity) :

Array.prototype.unique = function() {
    var o = {}, i, l = this.length, r = [];
    for(i=0; i<l;i+=1) o[this[i]] = this[i];
    for(i in o) r.push(o[i]);
    return r;

From Paul Irish's blog: improvement on JQuery .unique() :


    var _old = $.unique;

    $.unique = function(arr){

        // do the default behavior only if we got an array of elements
        if (!!arr[0].nodeType){
            return _old.apply(this,arguments);
        } else {
            // reduce the array to contain no dupes via grep/inArray
            return $.grep(arr,function(v,k){
                return $.inArray(v,arr) === k;

// in use..
var arr = ['first',7,true,2,7,true,'last','last'];
$.unique(arr); // ["first", 7, true, 2, "last"]

var arr = [1,2,3,4,5,4,3,2,1];
$.unique(arr); // [1, 2, 3, 4, 5]

I'm not sure why Gabriel Silveira wrote the function that way but a simpler form that works for me just as well and without the minification is:

Array.prototype.unique = function() {
  return this.filter(function(value, index, array) {
    return array.indexOf(value, index + 1) < 0;

or in CoffeeScript:

Array.prototype.unique = ->
  this.filter( (value, index, array) ->
    array.indexOf(value, index + 1) < 0

Finding unique Array values in simple method

function arrUnique(a){
  var t = [];
  for(var x = 0; x < a.length; x++){
    if(t.indexOf(a[x]) == -1)t.push(a[x]);
  return t;
arrUnique([1,4,2,7,1,5,9,2,4,7,2]) // [1, 4, 2, 7, 5, 9]

If you're okay with extra dependencies, or you already have one of the libraries in your codebase, you can remove duplicates from an array in place using LoDash (or Underscore).


If you don't have it in your codebase already, install it using npm:

npm install lodash

Then use it as follows:

import _ from 'lodash';
let idArray = _.uniq ([


[ 1, 2, 3 ]

It appears we have lost Rafael's answer, which stood as the accepted answer for a few years. This was (at least in 2017) the best-performing solution if you don't have a mixed-type array:

Array.prototype.getUnique = function(){
    var u = {}, a = [];
    for (var i = 0, l = this.length; i < l; ++i) {
        if (u.hasOwnProperty(this[i])) {
        u[this[i]] = 1;
return a;

If you do have a mixed-type array, you can serialize the hash key:

Array.prototype.getUnique = function() {
    var hash = {}, result = [], key; 
    for ( var i = 0, l = this.length; i < l; ++i ) {
        key = JSON.stringify(this[i]);
        if ( !hash.hasOwnProperty(key) ) {
            hash[key] = true;
    return result;

This has been answered a lot, but it didn't address my particular need.

Many answers are like this:

a.filter((item, pos, self) => self.indexOf(item) === pos);

But this doesn't work for arrays of complex objects.

Say we have an array like this:

const a = [
 { age: 4, name: 'fluffy' },
 { age: 5, name: 'spot' },
 { age: 2, name: 'fluffy' },
 { age: 3, name: 'toby' },

If we want the objects with unique names, we should use array.prototype.findIndex instead of array.prototype.indexOf:

a.filter((item, pos, self) => self.findIndex(v => === === pos);

To address the problem the other way around, it may be useful to have no duplicate while you load your array, the way Set object would do it but it's not available in all browsers yet. It saves memory and is more efficient if you need to look at its content many times.

Array.prototype.add = function (elem) {
   if (this.indexOf(elem) == -1) {


set = [];
[1,3,4,1,2,1,3,3,4,1].forEach(function(x) { set.add(x); });

Gives you set = [1,3,4,2]

strange this hasn't been suggested before.. to remove duplicates by object key (id below) in an array you can do something like this:

const uniqArray = array.filter((obj, idx, arr) => (
  arr.findIndex((o) => === === idx

The simplest answer is :

const array = [1, 1, 2, 2, 3, 5, 5, 2];
const uniqueArray = [ Set(array)];
console.log(uniqueArray); // [1, 2, 3, 5]