Remove Duplicates from JavaScript Array

"Smart" but naïve way

uniqueArray = a.filter(function(item, pos) {
    return a.indexOf(item) == pos;
})

Basically, we iterate over the array and, for each element, check if the first position of this element in the array is equal to the current position. Obviously, these two positions are different for duplicate elements.

Using the 3rd ("this array") parameter of the filter callback we can avoid a closure of the array variable:

uniqueArray = a.filter(function(item, pos, self) {
    return self.indexOf(item) == pos;
})

Although concise, this algorithm is not particularly efficient for large arrays (quadratic time).

Hashtables to the rescue

function uniq(a) {
    var seen = {};
    return a.filter(function(item) {
        return seen.hasOwnProperty(item) ? false : (seen[item] = true);
    });
}

This is how it's usually done. The idea is to place each element in a hashtable and then check for its presence instantly. This gives us linear time, but has at least two drawbacks:

• since hash keys can only be strings in Javascript, this code doesn't distinguish numbers and "numeric strings". That is, uniq([1,"1"]) will return just [1]
• for the same reason, all objects will be considered equal: uniq([{foo:1},{foo:2}]) will return just [{foo:1}].

That said, if your arrays contain only primitives and you don't care about types (e.g. it's always numbers), this solution is optimal.

The best from two worlds

An universal solution combines both approaches: it uses hash lookups for primitives and linear search for objects.

function uniq(a) {
    var prims = {"boolean":{}, "number":{}, "string":{}}, objs = [];

    return a.filter(function(item) {
        var type = typeof item;
        if(type in prims)
            return prims[type].hasOwnProperty(item) ? false : (prims[type][item] = true);
        else
            return objs.indexOf(item) >= 0 ? false : objs.push(item);
    });
}

sort | uniq

Another option is to sort the array first, and then remove each element equal to the preceding one:

function uniq(a) {
    return a.sort().filter(function(item, pos, ary) {
        return !pos || item != ary[pos - 1];
    })
}

Again, this doesn't work with objects (because all objects are equal for sort), unless a special compare function can be provided. Additionally, this method silently changes the original array as a side effect - not good! However, if your input is already sorted, this is the way to go (just remove sort from the above).

Unique by...

Sometimes it's desired to uniquify a list based on some criteria other than just equality, for example, to filter out objects that are different, but share some property. This can be done elegantly by passing a callback. This "key" callback is applied to each element, and elements with equal "keys" are removed. Since key is expected to return a primitive, hash table will work fine here:

function uniqBy(a, key) {
    var seen = {};
    return a.filter(function(item) {
        var k = key(item);
        return seen.hasOwnProperty(k) ? false : (seen[k] = true);
    })
}

A particularly useful key() is JSON.stringify which will remove objects that are physically different, but "look" the same:

a = [[1,2,3], [4,5,6], [1,2,3]]
b = uniqBy(a, JSON.stringify)
console.log(b) // [[1,2,3], [4,5,6]]

Libraries

Both underscore and Lo-Dash provide uniq methods. Their algorithms are basically similar to the first snippet above and boil down to this:

var result = [];
a.forEach(function(item) {
     if(result.indexOf(item) < 0) {
         result.push(item);
     }
});

This is quadratic, but there are nice additional goodies, like wrapping native indexOf, ability to uniqify by a key (iteratee in their parlance), and optimizations for already sorted arrays.

If you're using jQuery and can't stand anything without a dollar before it, it goes like this:

  $.uniqArray = function(a) {
        return $.grep(a, function(item, pos) {
            return $.inArray(item, a) === pos;
        });
  }

which is, again, a variation of the first snippet.

Performance

Function calls are expensive in Javascript, therefore the above solutions, as concise as they are, are not particularly efficient. For maximal performance, replace filter with a loop and get rid of other function calls:

function uniq_fast(a) {
    var seen = {};
    var out = [];
    var len = a.length;
    var j = 0;
    for(var i = 0; i < len; i++) {
         var item = a[i];
         if(seen[item] !== 1) {
               seen[item] = 1;
               out[j++] = item;
         }
    }
    return out;
}

This chunk of ugly code does the same as the snippet #3 above, but an order of magnitude faster:

function uniq(a) {
    var seen = {};
    return a.filter(function(item) {
        return seen.hasOwnProperty(item) ? false : (seen[item] = true);
    });
}

function uniq_fast(a) {
    var seen = {};
    var out = [];
    var len = a.length;
    var j = 0;
    for(var i = 0; i < len; i++) {
         var item = a[i];
         if(seen[item] !== 1) {
               seen[item] = 1;
               out[j++] = item;
         }
    }
    return out;
}

/////

var r = [0,1,2,3,4,5,6,7,8,9],
    a = [],
    LEN = 1000,
    LOOPS = 1000;

while(LEN--)
    a = a.concat(r);

var d = new Date();
for(var i = 0; i < LOOPS; i++)
    uniq(a);
document.write('<br>uniq, ms/loop: ' + (new Date() - d)/LOOPS)

var d = new Date();
for(var i = 0; i < LOOPS; i++)
    uniq_fast(a);
document.write('<br>uniq_fast, ms/loop: ' + (new Date() - d)/LOOPS)

Use Underscore.js

It's a library with a host of functions for manipulating arrays.

It's the tie to go along with jQuery's tux, and Backbone.js's suspenders.

_.uniq

_.uniq(array, [isSorted], [iterator]) Alias: unique
Produces a duplicate-free version of the array, using === to test object equality. If you know in advance that the array is sorted, passing true for isSorted will run a much faster algorithm. If you want to compute unique items based on a transformation, pass an iterator function.

Example

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

alert(_.uniq(names, false));

Note: Lo-Dash (an underscore competitor) also offers a comparable .uniq implementation.

Got tired of seeing all bad examples with for-loops or jQuery. Javascript has the perfect tools for this nowadays: sort, map and reduce.

Uniq reduce while keeping existing order

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

var uniq = names.reduce(function(a,b){
    if (a.indexOf(b) < 0 ) a.push(b);
    return a;
  },[]);

console.log(uniq, names) // [ 'Mike', 'Matt', 'Nancy', 'Adam', 'Jenny', 'Carl' ]

// one liner
return names.reduce(function(a,b){if(a.indexOf(b)<0)a.push(b);return a;},[]);

Faster uniq with sorting

There are probably faster ways but this one is pretty decent.

var uniq = names.slice() // slice makes copy of array before sorting it
  .sort(function(a,b){
    return a - b;
  })
  .reduce(function(a,b){
    if (a.slice(-1)[0] !== b) a.push(b); // slice(-1)[0] means last item in array without removing it (like .pop())
    return a;
  },[]); // this empty array becomes the starting value for a

// one liner
return names.slice().sort(function(a,b){return a - b}).reduce(function(a,b){if (a.slice(-1)[0] !== b) a.push(b);return a;},[]);

A single line version using array filter and indexOf functions:

arr = arr.filter (function (v, i, a) { return a.indexOf (v) == i });

You can always try putting it into an object, and then iterating through its keys:

function remove_duplicates(arr) {
    var obj = {};
    for (var i = 0; i < arr.length; i++) {
        obj[arr[i]] = true;
    }
    arr = [];
    for (var key in obj) {
        arr.push(key);
    }
    return arr;
}

Or, for an order-safe version, add it to an object and to a new array, and check to see if you've already added it to that object:

function remove_duplicates_safe(arr) {
    var obj = {};
    var arr2 = [];
    for (var i = 0; i < arr.length; i++) {
        if (!(arr[i] in obj)) {
            arr2.push(arr[i]);
            obj[arr[i]] = true;
        }
    }
    return arr2;

}

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

b = remove_duplicates(a);
// b:
// ["Adam", "Carl", "Jenny", "Matt", "Mike", "Nancy"]

c = remove_duplicates_safe(a);
// c:
// ["Mike", "Matt", "Nancy", "Adam", "Jenny", "Carl"]

The following is more than 80% faster than the jQuery method listed (see tests below). It is an answer from a similar question a few years ago, if I come across the person who originally proposed it I will post credit. Pure JS.

var temp = {};
  for (var i = 0; i < array.length; i++)
  temp[array[i]] = true;
  var r = [];
  for (var k in temp)
  r.push(k);
  return r;

My Test Case comparison: http://jsperf.com/remove-duplicate-array-tests

The top answers have complexity of O(n²), but this can be done with just O(n) by using an object as a hash:

function getDistinctArray(arr) {
    var dups = {};
    return arr.filter(function(el) {
        var hash = el.valueOf();
        var isDup = dups[hash];
        dups[hash] = true;
        return !isDup;
    });
}

This will work for strings, numbers, and dates. If your array contains complex objects (ie, they have to be compared with ===), the above solution won't work. You can get an O(n) implementation for objects by setting a flag on the object itself:

function getDistinctObjArray(arr) {
    var distinctArr = arr.filter(function(el) {
        var isDup = el.inArray;
        el.inArray = true;
        return !isDup;
    });
    distinctArr.forEach(function(el) {
        delete el.inArray;
    });
    return distinctArr;
}

You could also use the Array.unique() method from the JavaScript Lab library – or steal an idea from there.

However, the code there isn’t very well written, since it declares the unique() method as a property of the Array prototype, thus adding it to every Array, breaking the for...in functionality (because a for...in loop will always iterate over the unique variable, too).

Here is a simple answer to the question.

var names = ["Alex","Tony","James","Suzane", "Marie", "Laurence", "Alex", "Suzane", "Marie", "Marie", "James", "Tony", "Alex"];
var uniqueNames = [];

    for(var i in names){
        if(uniqueNames.indexOf(names[i]) === -1){
            uniqueNames.push(names[i]);
        }
    }

$(document).ready(function() {

    var arr1=["dog","dog","fish","cat","cat","fish","apple","orange"]

    var arr2=["cat","fish","mango","apple"]

    var uniquevalue=[];
    var seconduniquevalue=[];
    var finalarray=[];

    $.each(arr1,function(key,value){

       if($.inArray (value,uniquevalue) === -1)
       {
           uniquevalue.push(value)

       }

    });

     $.each(arr2,function(key,value){

       if($.inArray (value,seconduniquevalue) === -1)
       {
           seconduniquevalue.push(value)

       }

    });

    $.each(uniquevalue,function(ikey,ivalue){

        $.each(seconduniquevalue,function(ukey,uvalue){

            if( ivalue == uvalue)

            {
                finalarray.push(ivalue);
            }   

        });

    });
    alert(finalarray);
});

A slight modification of thg435's excellent answer to use a custom comparator:

function contains(array,obj) {
    for(var i =0;i<array.length;i++) {
        if(isEqual(array[i],obj))return true;
    }
    return false;
}
//comparator
function isEqual(obj1,obj2) {
    if(obj1.name==obj2.name) return true;
    return false;
}
function removeDuplicates(ary) {
    var arr = [];
    return ary.filter(function(x) {
        return !contains(arr,x) && arr.push(x);
    });
}

In EcmaScript 6, Set can be used to filter out duplicates. Then it can be converted back to an array using the spread operator.

var names = ["Mike","Matt","Nancy","Adam","Jenny","Nancy","Carl"],
    unique = [...Set(names)];

This is probably one of the fastest way to remove permanently the duplicates from an array 10x times faster than the most functions here.& 78x faster in safari

function toUnique(a,b,c){               //array,placeholder,placeholder
 b=a.length;while(c=--b)while(c--)a[b]!==a[c]||a.splice(c,1)
}

1. Test: http://jsperf.com/wgu
2. Demo: http://jsfiddle.net/46S7g/
3. More: http://stackoverflow.com/a/25082874/2450730

if you can't read the code above ask, read a javascript book or here are some explainations about shorter code. http://stackoverflow.com/a/21353032/2450730

Another method of doing this without writing much code is using the ES5 Object.keys-method:

var arrayWithDuplicates = ['a','b','c','d','a','c'],
    deduper = {};
arrayWithDuplicates.forEach(function (item) {
    deduper[item] = null;
});
var dedupedArray = Object.keys(deduper); // ["a", "b", "c", "d"]

Extracted in a function

function removeDuplicates (arr) {
    var deduper = {}
    arr.forEach(function (item) {
        deduper[item] = null;
    });
    return Object.keys(deduper);
}

go for this one ,

var uniqueArray = duplicateArray.filter(function(elem, pos) {
                        return duplicateArray.indexOf(elem) == pos;
                      }); 

Now uniqueArray contains no duplicates

Best method is using Underscore.js, it needs to do just one step. i.e _.uniq(list) . it will return list with uniq data.

Here is another approach using jQuery,

function uniqueArray(array){
  if ($.isArray(array)){
    var dupes = {}; var len, i;
    for (i=0,len=array.length;i<len;i++){
      var test = array[i].toString();
      if (dupes[test]) { array.splice(i,1); len--; i--; } else { dupes[test] = true; }
    }
  } 
  else {
    if (window.console) console.log('Not passing an array to uniqueArray, returning whatever you sent it - not filtered!');
      return(array);
  }
  return(array);
}

Author: William Skidmore

function removeDuplicates(inputArray) {
            var outputArray=new Array();

            if(inputArray.length>0){
                jQuery.each(inputArray, function(index, value) {
                    if(jQuery.inArray(value, outputArray) == -1){
                        outputArray.push(value);
                    }
                });
            }           
            return outputArray;
        }

If you don't want to include a whole library, you can use this one off to add a method that any array can use:

Array.prototype.uniq = function uniq() {
  return this.reduce(function(accum, cur) { 
    if (accum.indexOf(cur) === -1) accum.push(cur); 
    return accum; 
  }, [] );
}

["Mike","Matt","Nancy","Adam","Jenny","Nancy","Carl"].uniq()

If you're creating the array yourself, you can save yourself a loop and the extra unique filter by doing the check as you're inserting the data;

var values = [];
$.each(collection, function() {
    var x = $(this).value;
    if (!$.inArray(x, values)) {
        values.push(x);
    }
});

The easiest way to remove string duplicates is to use associative array and then iterate over the associative array to make the list/array back.

Like below:

var toHash = [];
var toList = [];

// add from ur data list to hash
$(data.pointsToList).each(function(index, Element) {
    toHash[Element.nameTo]= Element.nameTo;
});

// now convert hash to array
// don't forget the "hasownproperty" else u will get random results 
for (var key in toHash)  {
    if (toHash.hasOwnProperty(key)) { 
      toList.push(toHash[key]);
   }
}

Voila, now duplicates are gone!

The simplest way to remove a duplicate is to do a for loop and compare the elements that are not the same and push them into the new array

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

 var removeDublicate = function(arr){
 var result = []
 var sort_arr = arr.sort() //=> optional
 for (var i = 0; i < arr.length; i++) {
        if(arr[ i + 1] !== arr[i] ){
            result.push(arr[i])
        }
 };
  return result
}  
console.log(removeDublicate(array))
==>  ["Adam", "Carl", "Jenny", "Matt", "Mike", "Nancy"]

The following script returns a new array containing only unique values. It works on string and numbers. No requirement for additional libraries only vanilla JS.

Browser support:

Feature Chrome  Firefox (Gecko)     Internet Explorer   Opera   Safari
Basic support   (Yes)   1.5 (1.8)   9                   (Yes)   (Yes)

https://jsfiddle.net/fzmcgcxv/3/

var duplicates = ["Mike","Matt","Nancy","Adam","Jenny","Nancy","Carl","Mike","Mike","Nancy","Carl"]; 
var unique = duplicates.filter(function(elem, pos) {
    return duplicates.indexOf(elem) == pos;
  }); 
alert(unique);

I know Im a little late, but here is another option using jinqJs

See Fiddle

var result = jinqJs().from(["Mike","Matt","Nancy","Adam","Jenny","Nancy","Carl"]).distinct().select();

If by any chance you were using

D3.js

You could do

d3.set(["foo", "bar", "foo", "baz"]).values() ==> ["foo", "bar", "baz"]

https://github.com/mbostock/d3/wiki/Arrays#set_values