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How to get unique values in an array


Question

How can I get a list of unique values in an array? Do I always have to use a second array or is there something similar to java's hashmap in JavaScript?

I am going to be using JavaScript and jQuery only. No additional libraries can be used.

2015/03/04
1
171
3/4/2015 9:13:47 PM

Accepted Answer

Since I went on about it in the comments for @Rocket's answer, I may as well provide an example that uses no libraries. This requires two new prototype functions, contains and unique

Array.prototype.contains = function(v) {
  for (var i = 0; i < this.length; i++) {
    if (this[i] === v) return true;
  }
  return false;
};

Array.prototype.unique = function() {
  var arr = [];
  for (var i = 0; i < this.length; i++) {
    if (!arr.contains(this[i])) {
      arr.push(this[i]);
    }
  }
  return arr;
}

var duplicates = [1, 3, 4, 2, 1, 2, 3, 8];
var uniques = duplicates.unique(); // result = [1,3,4,2,8]

console.log(uniques);

For more reliability, you can replace contains with MDN's indexOf shim and check if each element's indexOf is equal to -1: documentation

2019/07/18
119
7/18/2019 4:09:22 PM


Here's a much cleaner solution for ES6 that I see isn't included here. It uses the Set and the spread operator: ...

var a = [1, 1, 2];

[... new Set(a)]

Which returns [1, 2]

2017/02/13

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+

2020/06/20

Now in ES6 we can use the newly introduced ES6 function

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

OR by Array spread syntax on iterables

let items = [1,1,1,1,3,4,5,2,23,1,4,4,4,2,2,2]; 
let uniqueItems = [...new Set(items)];

It will return the unique result.

[1, 3, 4, 5, 2, 23]
2020/05/14

Using EcmaScript 2016 you can simply do it like this.

 var arr = ["a", "a", "b"];
 var uniqueArray = Array.from(new Set(arr)); // Unique Array ['a', 'b'];

Sets are always unique, and using Array.from() you can convert a Set to an array. For reference have a look at the documentations.

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/from https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Set

2017/12/25

If you want to leave the original array intact,

you need a second array to contain the uniqe elements of the first-

Most browsers have Array.prototype.filter:

var unique= array1.filter(function(itm, i){
    return array1.indexOf(itm)== i; 
    // returns true for only the first instance of itm
});


//if you need a 'shim':
Array.prototype.filter= Array.prototype.filter || function(fun, scope){
    var T= this, A= [], i= 0, itm, L= T.length;
    if(typeof fun== 'function'){
        while(i<L){
            if(i in T){
                itm= T[i];
                if(fun.call(scope, itm, i, T)) A[A.length]= itm;
            }
            ++i;
        }
    }
    return A;
}
 Array.prototype.indexOf= Array.prototype.indexOf || function(what, i){
        if(!i || typeof i!= 'number') i= 0;
        var L= this.length;
        while(i<L){
            if(this[i]=== what) return i;
            ++i;
        }
        return -1;
    }
2012/06/28