Get all non-unique values (i.e.: duplicate/more than one occurrence) in an array

Get all non-unique values (i.e.: duplicate/more than one occurrence) in an array

I need to check a JavaScript array to see if there are any duplicate values. What’s the easiest way to do this? I just need to find what the duplicated values are – I don’t actually need their indexes or how many times they are duplicated.
I know I can loop through the array and check all the other values for a match, but it seems like there should be an easier way. Any ideas? Thanks!
Similar question:

Get all unique values in an array (remove duplicates)

Solutions/Answers:

Solution 1:

You could sort the array and then run through it and then see if the next (or previous) index is the same as the current. Assuming your sort algorithm is good, this should be less than O(n2):

var arr = [9, 9, 111, 2, 3, 4, 4, 5, 7];
var sorted_arr = arr.slice().sort(); // You can define the comparing function here. 
                                     // JS by default uses a crappy string compare.
                                     // (we use slice to clone the array so the
                                     // original array won't be modified)
var results = [];
for (var i = 0; i < sorted_arr.length - 1; i++) {
    if (sorted_arr[i + 1] == sorted_arr[i]) {
        results.push(sorted_arr[i]);
    }
}

console.log(results);

Solution 2:

If you want to elimate the duplicates, try this great solution:

function eliminateDuplicates(arr) {
  var i,
      len = arr.length,
      out = [],
      obj = {};

  for (i = 0; i < len; i++) {
    obj[arr[i]] = 0;
  }
  for (i in obj) {
    out.push(i);
  }
  return out;
}

Source:
http://dreaminginjavascript.wordpress.com/2008/08/22/eliminating-duplicates/

Solution 3:

This is my answer from the duplicate thread (!):

When writing this entry 2014 – all examples were for-loops or jQuery. Javascript has the perfect tools for this: sort, map and reduce.

Find duplicate items

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

var uniq = names
  .map((name) => {
    return {
      count: 1,
      name: name
    }
  })
  .reduce((a, b) => {
    a[b.name] = (a[b.name] || 0) + b.count
    return a
  }, {})

var duplicates = Object.keys(uniq).filter((a) => uniq[a] > 1)

console.log(duplicates) // [ 'Nancy' ]

More functional syntax:

@Dmytro-Laptin pointed out some code code be removed. This is a more compact version of the same code. Using some ES6 tricks and higher order functions:

const names = ['Mike', 'Matt', 'Nancy', 'Adam', 'Jenny', 'Nancy', 'Carl']

const count = names =>
  names.reduce((a, b) => ({ ...a,
    [b]: (a[b] || 0) + 1
  }), {}) // don't forget to initialize the accumulator

const duplicates = dict =>
  Object.keys(dict).filter((a) => dict[a] > 1)

console.log(count(names)) // { Mike: 1, Matt: 1, Nancy: 2, Adam: 1, Jenny: 1, Carl: 1 }
console.log(duplicates(count(names))) // [ 'Nancy' ]

Solution 4:

Find duplicate values in an array

This should be one of the shortest ways to actually find duplicate values in an array. As specifically asked for by the OP, this does not remove duplicates but finds them.

var input = [1, 2, 3, 1, 3, 1];

var duplicates = input.reduce(function(acc, el, i, arr) {
  if (arr.indexOf(el) !== i && acc.indexOf(el) < 0) acc.push(el); return acc;
}, []);

document.write(duplicates); // = 1,3 (actual array == [1, 3])

This doesn’t need sorting or any third party framework. It also doesn’t need manual loops. It works with every value indexOf() (or to be clearer: the strict comparision operator) supports.

Because of reduce() and indexOf() it needs at least IE 9.

Solution 5:

You can add this function, or tweak it and add it to Javascript’s Array prototype:

Array.prototype.unique = function () {
    var r = new Array();
    o:for(var i = 0, n = this.length; i < n; i++)
    {
        for(var x = 0, y = r.length; x < y; x++)
        {
            if(r[x]==this[i])
            {
                alert('this is a DUPE!');
                continue o;
            }
        }
        r[r.length] = this[i];
    }
    return r;
}

var arr = [1,2,2,3,3,4,5,6,2,3,7,8,5,9];
var unique = arr.unique();
alert(unique);

Solution 6:

UPDATED: The following uses an optimized combined strategy. It optimizes primitive lookups to benefit from hash O(1) lookup time (running unique on an array of primitives is O(n)). Object lookups are optimized by tagging objects with a unique id while iterating through so so identifying duplicate objects is also O(1) per item and O(n) for the whole list. The only exception is items that are frozen, but those are rare and a fallback is provided using an array and indexOf.

var unique = function(){
  var hasOwn = {}.hasOwnProperty,
      toString = {}.toString,
      uids = {};

  function uid(){
    var key = Math.random().toString(36).slice(2);
    return key in uids ? uid() : uids[key] = key;
  }

  function unique(array){
    var strings = {}, numbers = {}, others = {},
        tagged = [], failed = [],
        count = 0, i = array.length,
        item, type;

    var id = uid();

    while (i--) {
      item = array[i];
      type = typeof item;
      if (item == null || type !== 'object' && type !== 'function') {
        // primitive
        switch (type) {
          case 'string': strings[item] = true; break;
          case 'number': numbers[item] = true; break;
          default: others[item] = item; break;
        }
      } else {
        // object
        if (!hasOwn.call(item, id)) {
          try {
            item[id] = true;
            tagged[count++] = item;
          } catch (e){
            if (failed.indexOf(item) === -1)
              failed[failed.length] = item;
          }
        }
      }
    }

    // remove the tags
    while (count--)
      delete tagged[count][id];

    tagged = tagged.concat(failed);
    count = tagged.length;

    // append primitives to results
    for (i in strings)
      if (hasOwn.call(strings, i))
        tagged[count++] = i;

    for (i in numbers)
      if (hasOwn.call(numbers, i))
        tagged[count++] = +i;

    for (i in others)
      if (hasOwn.call(others, i))
        tagged[count++] = others[i];

    return tagged;
  }

  return unique;
}();

If you have ES6 Collections available, then there is a much simpler and significantly faster version. (shim for IE9+ and other browsers here: https://github.com/Benvie/ES6-Harmony-Collections-Shim)

function unique(array){
  var seen = new Set;
  return array.filter(function(item){
    if (!seen.has(item)) {
      seen.add(item);
      return true;
    }
  });
}