Get all non-unique values i e duplicate more than one occurrence in an array

Question

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   Similar question    Get all unique values in a JavaScript array  remove duplicates

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Surprised no one posted this solution    lt  DOCTYPE html gt   lt html gt   lt head gt   lt meta charset utf-8   gt   lt title gt   lt  title gt   lt  head gt   lt body gt     lt script gt         var list    100 33 45 54 9 12 80 100          var newObj              var newArr               for var i 0  i lt list length  i               newObj list i     i                                   for var j in newObj               newArr push j                      console log newArr      lt  script gt   lt  body gt   lt  html gt

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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  x000D   x000D  var names     Mike    Matt    Nancy    Adam    Jenny    Nancy    Carl    var uniq   names    map  name    gt        return         count  1        name  name               reduce  a  b    gt        a b name     a b name     0    b count     return a           var duplicates   Object keys uniq  filter  a    gt  uniq a   gt  1   console log duplicates        Nancy    x000D   x000D   x000D   More functional syntax   Dmytro-Laptin pointed out some code that can be removed  This is a more compact version of the same code  Using some ES6 tricks and higher order functions   x000D   x000D  const names     Mike    Matt    Nancy    Adam    Jenny    Nancy    Carl    const count   names   gt    names reduce  a  b    gt        a       b    a b     0    1              don t forget to initialize the accumulator  const duplicates   dict   gt    Object keys dict  filter  a    gt  dict a   gt  1   console log count names        Mike  1  Matt  1  Nancy  2  Adam  1  Jenny  1  Carl  1   console log duplicates count names          Nancy    x000D   x000D   x000D

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Based on  bluemoon but shorter  returns all duplicates exactly once  function checkDuplicateKeys arr        const counts          return arr filter  item    gt            counts item    counts item     1         if  counts item        2  return true               1 2 2 2 2 2 2    gt   1 2       dog    dog    cat     gt    dog

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From Raphael Montanaro answer  it can improve to use with array object item as follows   function eliminateDuplicates arr      var len   arr length        out             obj          for  var key  i 0  i  lt  len  i          key   JSON stringify arr i        obj key     obj key     obj key    1   1        for  var key in obj        out push JSON parse key          return  out  obj       Note  You need to use JSON library for browser that s not supported JSON

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x000D   x000D  var arr    2  1  2  2  4  4  2  5   x000D   x000D  function returnDuplicates arr    x000D    return arr reduce function dupes  val  i    x000D      if  arr indexOf val      i  amp  amp  dupes indexOf val      -1    x000D        dupes push val   x000D        x000D      return dupes  x000D            x000D    x000D   x000D  alert returnDuplicates arr    x000D   x000D   x000D    This function avoids the sorting step and uses the reduce   method to push duplicates to a new array if it doesn t already exist in it

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I am trying to improve the answer from  swilliams  this will return an array without duplicates      arrays for testing var arr    9  9  111  2  3  4  4  5  7       ascending order var sorted arr   arr sort function a b  return a-b       var arr length   arr length  var results       if arr length       if arr length    1           results   arr       else          for  var i   0  i  lt  arr length - 1  i                  if  sorted arr i   1     sorted arr i                     results push sorted arr i                                 for last element             if  i    arr length - 2                   results push sorted arr i 1                                     alert results

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ES6 offers the Set data structure which is basically an array that doesn t accept duplicates  With the Set data structure  there s a very easy way to find duplicates in an array  using only one loop    Here s my code  function findDuplicate arr    var set   new Set    var duplicates   new Set      for  let i   0  i lt  arr length  i           var size   set size       set add arr i         if  set size     size             duplicates add arr i                return duplicates

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This is how I implemented it with map  It should run in O n  time and should kinda be easy to gasp    x000D   x000D      var first array  1 1 2 3 4 4 5 6   x000D      var find dup new Map  x000D   x000D      for  const iterator of first array    x000D                 if present value   x000D              if find dup has iterator     x000D                  find dup set iterator find dup get iterator  1    x000D               else  x000D                 else add it x000D                  find dup set iterator 1   x000D                x000D            x000D      console log find dup get 2    x000D   x000D   x000D    Then you can find dup get key  to find if it has duplicates  it should give   1

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I prefer the function way of doing this   function removeDuplicates links        return   reduce links  function list  elem             if  list indexOf elem     -1                list push elem                        return list                  This uses underscore  but Array has a reduce function  too

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Similar to a few other answers  but I used forEach   to make it a bit prettier   function find duplicates data         var track           var duplicates            data forEach function  item             track item    track item    true   duplicates push item                return duplicates      If a value is duplicated more than once  all its duplicates are returned  like so   find duplicates   foo    foo    bar    bar    bar        returns   foo    bar    bar     This might be what you want  otherwise you just have to follow with an  unique  filtering

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This should get you what you want  Just the duplicates   function find duplicates arr      var len arr length        out           counts        for  var i 0 i lt len i          var item   arr i       counts item    counts item   gt   1   counts item    1   1      if  counts item      2          out push item                return out     find duplicates   one  2 3 4 4 4 5 6 7 7 7  pig   one        - gt    one  4 7  in no particular order

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I think the below is the easiest and fastest O n  way to accomplish exactly what you asked   function getDuplicates  arr       var i  value    var all         var duplicates          for  i 0  i lt arr length  i           value   arr i       if  all value            duplicates push  value          all value    false        else if  typeof all value      undefined            all value    true               return duplicates      Or for ES5 or greater   function getDuplicates  arr       var all         return arr reduce function  duplicates  value         if  all value            duplicates push value         all value    false        else if  typeof all value      undefined            all value    true            return duplicates

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Modifying  RaphaelMontanaro s solution  borrowing from  Nosredna s blog  here is what you could do if you just want to identify the duplicate elements from your array   function identifyDuplicatesFromArray arr            var i          var len   arr length          var obj               var duplicates                for  i   0  i  lt  len  i                   if   obj arr i                       obj arr i                                     else                               duplicates push arr i                                     return duplicates          Thanks for the elegant solution   Nosredna

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one liner simple way  x000D   x000D  var arr    9 1 2 4 3 4 9  console log arr filter  ele indx   gt indx   arr indexOf ele      get the duplicates console log arr filter  ele indx   gt indx   arr indexOf ele      remove the duplicates x000D   x000D   x000D

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Returns duplicates and preserves data type   With O 4n  performance  const dupes   arr   gt      const map   arr reduce  map  curr    gt        return  map set curr   map get curr     0    1   map       new Map        return Array from map  filter   key  val    gt  val  gt  1  map   key  val     gt  key      With O 2n  performance  const dupes   arr   gt      const map   arr reduce  map  curr    gt        return  map set curr   map get curr     0    1   map       new Map        const dupes          for  let  key  val  of map entries          if  val  gt  1  dupes  push key         return dupes

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Simplest way to fetch duplicates repeated values from array string     x000D   x000D  function getDuplicates param    x000D    var duplicates      x000D   x000D    for  var i   0  i  lt  param length  i      x000D      var char   param i  x000D      if  duplicates char     x000D        duplicates char    x000D        else   x000D        duplicates char    1 x000D        x000D      x000D    return duplicates x000D    x000D   x000D  console log getDuplicates  aeiouaeiou     x000D  console log getDuplicates   a    e    i    o    u    a    e      x000D  console log getDuplicates  1  2  3  4  5  1  1  2  3     x000D   x000D   x000D

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The Prototype library has a uniq function  which returns the array without the dupes  That s only half of the work though

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Following logic will be easier and faster      Param data Array that is the source      Return   Array that have the duplicate entries findDuplicates data  Array lt any gt    Array lt any gt            return Array from new Set data   filter  value    gt  data indexOf value      data lastIndexOf value              Advantages      Single line  -P All inbuilt data structure helping in improving the efficiency Faster   Description of Logic       Converting to set to remove all duplicates Iterating through the set values With each set value check in the source array for the condition  values first index is not equal to the last index       Then inferred as duplicate else it is  unique    Note  map   and filter   methods are efficient and faster

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I have just figured out a simple way to achieve this using an Array filter   x000D   x000D      var list    9  9  111  2  3  4  4  5  7   x000D       x000D         Filter 1  to find all duplicates elements x000D      var duplicates   list filter function value index self    x000D         return self indexOf value      self lastIndexOf value   amp  amp  self indexOf value      index  x000D          x000D       x000D      console log duplicates   x000D   x000D   x000D

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This is one of the simple ES5 solution I could think of -   function duplicates arr      var duplicatesArr             uniqueObj          for  var i   0  i  lt  arr length  i          if  uniqueObj hasOwnProperty arr i    amp  amp  duplicatesArr indexOf  arr i        -1          duplicatesArr push  arr i               else         uniqueObj  arr i      true               return duplicatesArr       Input Arr   1 1 2 2 2 1 3 4 5 3        OutPut Arr   1 2 3

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function GetDuplicates arr        var i   0  m           return arr filter function  n            return  m n     arr indexOf n  m n      i

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x000D   x000D  var arr    4 5 1 1 2 3 4 4 7 5 2 6 10 9   x000D  var sorted arr   arr sort    x000D  var len   arr length  x000D  var results       x000D  for  var i   0  i  lt  len  i      x000D    if  sorted arr i   1      sorted arr i     x000D      results push sorted arr i    x000D      x000D    x000D  document write results   x000D   x000D   x000D

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Here s the simplest solution I could think of   x000D   x000D  const arr    -1  2  2  2  0  0  0  500  -1   a    a    a    const filtered   arr filter  el  index    gt  arr indexOf el      index       gt  filtered     2  2  0  0  -1   a    a     const duplicates       new Set filtered    console log duplicates       gt    2  0  -1   a    x000D   x000D   x000D   That s it  Note   It works with any numbers including 0  strings and negative numbers e g  -1 - Related question   Get all unique values in a JavaScript array  remove duplicates   The original array arr is preserved  filter returns the new array instead of modifying the original   The filtered array contains all duplicates  it can also contain more than 1 same value  e g  our filtered array here is   2  2  0  0  -1   a    a      If you want to get only values that are duplicated  you don t want to have multiple duplicates with the same value  you can use     new Set filtered    ES6 has an object Set which can store only unique values    Hope this helps

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You can make use of the sort  filter and sets to do that    x000D   x000D  var numbers    1 2 3 4 5 6 7 8 1 2 3 4 5 3 3 4   x000D  var numbersSorted   numbers sort    x000D  let result   numbers filter  e  i    gt  numbers i     numbers i 1    x000D  result       new Set result    x000D  console log result   x000D   x000D   x000D

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Here is one implemented using sort   and JSON stringify    https   gist github com korczis 7598657  function removeDuplicates vals        var res           var tmp   vals sort         for  var i   0  i  lt  tmp length  i              res push tmp i                        while  JSON stringify tmp i      JSON stringify tmp i   1                  i                         return res    console log removeDuplicates  1 2 3 4 5 4 3 3 2 1

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The following function  a variation of the eliminateDuplicates function already mentioned  seems to do the trick  returning test2 1 7 5 for the input   test    test2    test2   1  1  1  2  3  4  5  6  7  7  10  22  43  1  5  8   Note that the problem is stranger in JavaScript than in most other languages  because a JavaScript array can hold just about anything  Note that solutions that use sorting might need to provide an appropriate sorting function--I haven t tried that route yet   This particular implementation works for  at least  strings and numbers   function findDuplicates arr        var i          len arr length          out             obj          for  i 0 i lt len i              if  obj arr i      null                if   obj arr i                      out push arr i                    obj arr i     1                          else               obj arr i     0                                  return out

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Printing duplicate values    x000D   x000D   var arr    1 2 3 4 13 2 3 4 3 4   x000D   x000D         non unique Printing  x000D      function nonUnique arr   x000D      var result       x000D      for var i  0 i lt arr length i     x000D          if arr indexOf arr i  i 1   gt  -1   x000D              result push arr i    x000D            x000D        x000D      console log result   x000D       nonUnique arr   x000D   x000D         unique Printing x000D      function uniqueDuplicateVal arr   x000D         var result       x000D         for var i  0 i lt arr length i     x000D          if arr indexOf arr i  i 1   gt  -1   x000D            if result indexOf arr i       -1    x000D               result push arr i    x000D              x000D            x000D               x000D        x000D      uniqueDuplicateVal arr  x000D   x000D   x000D

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var a   1  2 2 3 3 4 4 4   var m     var n       a forEach function e      if m indexOf e     -1        m push e    else if n indexOf e     -1       n push e

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I feel like the simplest solution would to just use indexOf  full example of pushing only unique elements to an array   var arr     a   b   c   d   a   b   c   d    var newA       for var i   0  i  lt  arr length  i         if newA indexOf arr i       -1           newA push arr i

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Yet another way by using underscore  Numbers is the source array and dupes has possible duplicate values   var itemcounts     countBy numbers  function  n    return n      var dupes     reduce itemcounts  function  memo  item  idx        if  item  gt  1          memo push idx       return memo

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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    x000D   x000D  var input    1  2  3  1  3  1   x000D   x000D  var duplicates   input reduce function acc  el  i  arr    x000D    if  arr indexOf el      i  amp  amp  acc indexOf el   lt  0  acc push el   return acc  x000D          x000D   x000D  document write duplicates        1 3  actual array     1  3   x000D   x000D   x000D    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

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Here is a very light and easy way   var codes   dc 1 split       var i   codes length  while  i--      if  codes indexOf codes i      i        codes splice i 1

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This will return duplicates from an Array as an Array of duplicates    x000D   x000D      const duplicates   function arr    x000D           let try moving in pairs   maybe that will work x000D        let dups   new Set    x000D            r      x000D        arr sort   x000D        arr reduce  pv  cv    gt    x000D          if  pv     cv    x000D            dups add pv  x000D            x000D          return cv x000D           x000D        for  let m of dups values      x000D          r push m  x000D          x000D        return r x000D        x000D       x000D      console log duplicates  1 3 5 6 7 4 4 5 1 4 6 3 8 9 5 0    x000D   x000D   x000D

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Using  includes  to test if the element already exists    x000D   x000D  var arr    1  1  4  5  5   darr       duplicates       x000D   x000D  for var i   0  i  lt  arr length  i     x000D    if darr includes arr i    amp  amp   duplicates includes arr i    x000D      duplicates push arr i   x000D    else x000D      darr push arr i    x000D    x000D   x000D  console log duplicates   x000D   lt h3 gt Array with duplicates lt  h3 gt  x000D   lt p gt  1  1  4  5  5  lt  p gt  x000D   lt h3 gt Array with distinct elements lt  h3 gt  x000D   lt p gt  1  4  5  lt  p gt  x000D   lt h3 gt duplicate values are lt  h3 gt  x000D   lt p gt  1  5  lt  p gt  x000D   x000D   x000D

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x000D   x000D    find duplicates  x000D    sort  then reduce - concat values equal previous element  skip others x000D   x000D    input x000D  var a    1  2  3  1  2  1  2  x000D   x000D    short version  x000D  var duplicates   a sort   reduce  d  v  i  a    gt  i  amp  amp  v     a i - 1    d concat v    d      x000D  console log duplicates      1  1  2  2  x000D   x000D    readable version  x000D  var duplicates   a sort   reduce  output  element  index  input    gt    x000D    if   index  gt  0   amp  amp   element     input index - 1    x000D      return output concat element  x000D    return output x000D         x000D  console log duplicates      1  1  2  2  x000D   x000D   x000D

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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   amp  amp  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

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var isUnique   true        for  var i  0  i lt  targetItems length  i              var itemValue     targetItems i   val            if  targetListValues indexOf itemValue   gt   0              isUnique   false             break                  targetListValues push itemValue           if   isUnique                raise any error msg           return false

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This is more advanced function based on number of occurences   function getMostDuplicated arr  count          const result             arr forEach  item    gt            let i   0          arr forEach  checkTo    gt              if  item     checkTo                i                if  i     count  amp  amp  result indexOf item      -1                  result push item                                                         return result     arr    1 1 1 2 5 67 3 2 3 2 3 1 2 3 4 1 4   result   getMostDuplicated arr  5      result is 1 result   getMostDuplicated arr  2      result 1  2  3  4 console log result

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Very simple way    x000D   x000D  function getDuplicateValues someArray    x000D   const duplicateValues   new Set     x000D   const check   new Set     x000D   someArray forEach v   gt    x000D    if  check has v     x000D     duplicateValues add v  x000D      else   x000D     check add v  x000D      x000D      x000D   return Array from duplicateValues   x000D    x000D   x000D  const result   getDuplicateValues   coffee    soda    water    juice    water    water    coffee    x000D   x000D  repeated values textContent   JSON stringify result  null        x000D   lt pre id  repeated values  gt  lt  pre gt  x000D   x000D   x000D

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Here we output the duplicates just once per dupe    x000D   x000D  var arr    9  9  9  9  111  2  3  4  4  5  7   x000D  arr sort     x000D   x000D  var results       x000D  for  var i   0  i  lt  arr length - 1  i      x000D      if  arr i   1     arr i     x000D          results push arr i    x000D        x000D    x000D   x000D  results   Array from new Set results   x000D   x000D  console log results   x000D   x000D   x000D

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This answer might also be helpful  it leverages js reduce operator method to remove duplicates from array    x000D   x000D  const result    1  2  2  3  3  3  3  reduce  x  y    gt  x includes y    x       x  y        x000D   x000D  console log result   x000D   x000D   x000D

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You can use filter method and indexOf   to get all the duplicate values function duplicate arr        return duplicateArray   arr filter  item  index    gt  arr indexOf item      index      arr indexOf item  will always return the first index at which a given element can be found

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The accepted answer is the most perfect one but as some users has pointed that for cases where an element is repeated more than 2 times it will gives us the array with repeated elements  This solution covers that scenarios too   const peoples        id  1  name  quot Arjun quot       id  2  name  quot quinze quot       id  3  name  quot catorze quot       id  1  name  quot Arjun quot       id  4  name  quot dezesseis quot       id  1  name  quot Arjun quot       id  2  name  quot quinze quot       id  3  name  quot catorzee quot       function repeated ppl      const newppl   ppl slice   sort  a b    gt  a id -b id      let rept         for let i   0  i  lt  newppl length-1   i         if  newppl i 1  id    newppl i  id         rept push newppl i 1                 return     new Set rept map el   gt  el id    map rid   gt       rept find el   gt  el id     rid           repeated peoples

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Shortest vanilla JS    1 1 2 2 2 3  filter  v i a    gt  a indexOf v      i      1  2  2

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The quickest way to solve is is actually with a flag   x000D   x000D  var values    4 2 3 1 4  x000D   x000D     solution x000D  const checkDuplicate   list   gt    x000D    var hasDuplicate   false  x000D    list sort   sort  a  b    gt    x000D      if  a     b  hasDuplicate   true x000D       x000D    return hasDuplicate x000D    x000D   x000D  console log checkDuplicate values   x000D   x000D   x000D

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With ES6  or using Babel or Typescipt  you can simply do   var duplicates   myArray filter i   gt  myArray filter ii   gt  ii     i  length  gt  1     https   es6console com j58euhbt

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var array     a    b    c    a     function unique array        var unique arr           array forEach function i  e            if  unique arr indexOf i    -1  unique arr push i               return unique arr    console log unique array

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ES5 only  i e   it needs a filter   polyfill for IE8 and below    var arrayToFilter     4  5  5  5  2  1  3  1  1  2  1  3     arrayToFilter      sort        filter  function me i arr          return  i   0       me     arr i-1

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There is a really simple way to solve this   If you use the newish  Set  javascript command   Set can take an array as input and output a new  Set  that only contains unique values   Then by comparing the length of the array and the  size  property of the set you can see if they differ   If they differ it must be due to a duplicate entry    x000D   x000D  var array1     value1   value2   value3   value1       contains duplicates x000D  var array2     value1   value2   value3   value4       unique values x000D   x000D  console log  array1 contains duplicates       containsDuplicates array1    x000D  console log  array2 contains duplicates       containsDuplicates array2    x000D   x000D   x000D  function containsDuplicates passedArray    x000D    let mySet   new Set passedArray   x000D    if  mySet size     passedArray length    x000D      return true  x000D      x000D    return false  x000D    x000D   x000D   x000D    If you run the above snippet you will get this output   array1 contains duplicates   true  array2 contains duplicates   false

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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     x000D   x000D  const findDuplicates    arr    gt    x000D    let sorted arr   arr slice   sort       You can define the comparing function here   x000D       JS by default uses a crappy string compare  x000D        we use slice to clone the array so the x000D       original array won t be modified  x000D    let results       x000D    for  let i   0  i  lt  sorted arr length - 1  i      x000D      if  sorted arr i   1     sorted arr i     x000D        results push sorted arr i    x000D        x000D      x000D    return results  x000D    x000D   x000D  let duplicatedArray    9  9  111  2  3  4  4  5  7   x000D  console log  The duplicates in   duplicatedArray  are   findDuplicates duplicatedArray      x000D   x000D   x000D    In case  if you are to return as a function for duplicates  This is for similar type of case   Reference  https   stackoverflow com a 57532964 8119511

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http   jsfiddle net vol7ron gfJ28   var arr      hello   goodbye   foo   hello   foo   bar  1 2 3 4 5 6 7 8 9 0 1 2 3   var hash           build hash for  var n arr length  n--        if  typeof hash arr n        undefined   hash arr n            hash arr n   push n          work with compiled hash  not necessary  var duplicates       for  var key in hash       if  hash hasOwnProperty key   amp  amp  hash key  length  gt  1           duplicates push key               alert duplicates      The result will be the hash array  which will contain both a unique set of values and the position of those values   So if there are 2 or more positions  we can determine that the value has a duplicate   Thus  every place hash  lt value gt   length  gt  1  signifies a duplicate  hash  hello   will return  0 3  because  hello  was found in node 0 and 3 in arr       Note  the length of  0 3  is what s used to determine if it was a duplicate  Using for var key in hash   if  hash hasOwnProperty key    alert key       will alert each unique value

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Here is mine simple and one line solution    It searches not unique elements first  then makes found array unique with the use of Set   So we have array of duplicates in the end    x000D   x000D  var array    1  2  2  3  3  4  5  6  2  3  7  8  5  22  1  2  511  12  50  22   x000D   x000D  console log     new Set  x000D    array filter  value  index  self    gt  self indexOf value      index    x000D     x000D   x000D   x000D

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Find non-unique values from 3 arrays  or more   ES2015  x000D   x000D                                   var arr     1 2 2 3 3 4 5 6 2 3 7 8 5 22       arr2    1 2 511 12 50       arr3    22 0       merged      nonUnique      Combine all the arrays to a single one merged   arr concat arr2  arr3      create a new  dirty  Array with only the non-unique items nonUnique   merged filter  item i    gt  merged includes item  i 1       Cleanup - remove duplicate  amp  empty items items  nonUnique       new Set nonUnique    console log nonUnique  x000D   x000D   x000D    PRE-ES2015  In the below example I chose to superimpose a unique method on top of the Array prototype  allowing access from everywhere and has more  quot declarative quot  syntax  I do not recommend this approach on large projects  since it might very well collide with another method with the same custom name   x000D   x000D  Array prototype unique   function          var arr   this sort    i arr length     input must be sorted for this to work     while i--        arr i      arr i-1   amp  amp  arr splice i 1     remove duplicate item     return arr    Array prototype nonunique   function          var arr   this sort    i arr length  res          input must be sorted for this to work     while i--        arr i      arr i-1   amp  amp   res indexOf arr i      -1   amp  amp  res push arr i        return res                                    var arr     1 2 2 3 3 4 5 6 2 3 7 8 5 22       arr2    1 2 511 12 50       arr3    22 0          merge all arrays  amp  call custom Array Prototype -  unique      unique   arr concat arr2  arr3  unique        nonunique   arr concat arr2  arr3  nonunique    console log unique          1 12 2 22 3 4 5 50 511 6 7 8  console log nonunique       1 12 2 22 3 4 5 50 511 6 7 8  x000D   x000D   x000D

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here is a small simple snippet to  find unique and duplicate values with out sorting and two loops    x000D   x000D  var  unique   function  arr    x000D      var h       t       x000D      arr forEach function  n    x000D          if  h indexOf n     -1  x000D              h push n   x000D          else t push n   x000D          x000D      return  h  t   x000D    x000D  var result    unique   test  1 4 2 34 6 21 3 4  test   prince   th  34    x000D  console log  Has duplicate values        result 1  length  gt  0      and you can check count of duplicate values x000D  console log result 0     unique values x000D  console log result 1     duplicate values x000D   x000D   x000D

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var arr     a   b   c   a     arr filter   item   index     gt      console log item   index   arr indexOf item    arr indexOf  item      index   return index    arr indexOf item

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Magic  a filter   t     e  gt   1- t e    t e  0         O n  performance  we assume your array is in a and it contains elements that can be cast  toString   in unique way  which is done implicity by JS in t e     e g numbers  4 5 4   strings   aa   bb   aa    arraysNum   1 2 3    43 2 3   1 2 3    Explanation here  unique values here   x000D   x000D  var a1     2  17    2  17    2  17    1  12    5  9    1  12    6  2    1  12    x000D  var a2     Mike    Adam   Matt    Nancy    Adam    Jenny    Nancy    Carl    x000D  var a3    5 6 4 9 2 3 5 3 4 1 5 4 9   x000D   x000D  let nd    a    gt  a filter  t    e  gt   1- t e    t e  0      x000D   x000D   x000D     Print x000D  let c  x   gt  console log JSON stringify x                 x000D  c  nd a1     x000D  c  nd a2     x000D  c  nd a3     x000D   x000D   x000D

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Fast and elegant way using es6 object destructuring and reduce  It runs in O n   1 iteration over the array  and doesn t repeat values that appear more than 2 times   x000D   x000D  const arr     hi    hi    hi    bye    bye    asd   x000D  const   x000D    dup x000D      arr reduce  x000D     acc  curr    gt    x000D      acc items curr    acc items curr    acc items curr     1   1 x000D      if  acc items curr      2  acc dup push curr  x000D      return acc x000D         x000D      items      x000D      dup     x000D       x000D    x000D   x000D  console log dup  x000D       hi    bye   x000D   x000D   x000D

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using underscore js  function hasDuplicate arr       return  arr length      uniq arr  length

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I did not like most answers    Why  Too complicated  too much code  inefficient code and many do not answer the question  which is to find the duplicates  and not to give an array without the duplicates     Next function returns all duplicates   function GetDuplicates arr      var i  out     obj       for  i 0  i  lt  arr length  i         obj arr i      undefined   obj arr i        out push arr i      return out        Because most of the time it is of no use to return ALL duplicates  but just to tell which duplicate values exist  In that case you return an array with unique duplicates  -   function GetDuplicates arr      var i  out     obj       for  i 0  i  lt  arr length  i        obj arr i      undefined   obj arr i        out push arr i      return GetUnique out      function GetUnique arr      return   grep arr  function elem  index        return index      inArray elem  arr             Maybe somebody else thinks the same

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The indexOf method of the Array object is useful for comparing array items  IE is the only major browser that does not natively support it  but it is easy to implement      Array prototype indexOf  Array prototype indexOf    function what  i       i  i    0      var L  this length      while i lt L           if this i     what  return i            i            return -1     function getarrayduplicates arg       var itm  A  arg slice 0  arg length   dups          while A length           itm  A shift            if A indexOf itm    -1  amp  amp  dups indexOf itm    -1               dups dups length   itm                      return dups      var a1   1  22  3  2  2  3  3  4  1  22  7  8  9    alert getarrayduplicates a1     For very large arrays  it can be faster to remove the duplicates from the array as they are found  so that they will not be looked at again   function getarrayduplicates arg       var itm  A  arg slice 0  arg length   dups          while A length           itm  A shift            if A indexOf itm    -1               dups dups length   itm              while A indexOf itm    -1                   A splice A indexOf itm   1                                     return dups

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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  lt  n  i                  for var x   0  y   r length  x  lt  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

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var a    324 3 32 5 52 2100 1 20 2 3 3 2 2 2 1 1 1  sort    a filter function v i o  return i amp  amp v   o i-1  v 0       or when added to the prototyp chain of Array    copy and paste  without error handling Array prototype unique       function   return this sort   filter function v i o  return i amp  amp v   o i-1  v 0        See here  https   gist github com 1305056

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We will use Javascript ES6 Functionality to do magic   var arr    9  9  111  2  3  4  4  5  7   const filtered   arr filter  value  index    gt     return arr indexOf value   gt   index       console log filtered     https   jsfiddle net 97Lxupnz

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This is probably one of the fastest way to remove permanently the duplicates from an array 10x times faster than the most functions here  amp  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    var array  1 2 3 4 5 6 7 8 9 0 1 2 1   toUnique array   console log array      Test  http   jsperf com wgu Demo  http   jsfiddle net 46S7g  More  https   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  https   stackoverflow com a 21353032 2450730  EDIT As stated in the comments this function does return an array with uniques  the question however asks to find the duplicates  in that case a simple modification to this function allows to push the duplicates into an array  then using the previous function toUnique removes the duplicates of the duplicates    function theDuplicates a b c d    array placeholder placeholder  b a length d      while c --b while c-- a b    a c   d push a splice c 1     var array  1 2 3 4 5 6 7 8 9 0 1 2 1    toUnique theDuplicates array

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This should be one of the shortest and easiest ways to actually find duplicate values in an array    x000D   x000D  var arr    1 2 3 4 5 6 7 8 1 2 3 4 5 3 3 4   x000D  var data   arr filter function item index arr   x000D    return arr indexOf item     arr lastIndexOf item   amp  amp  arr indexOf item     index  x000D     x000D   x000D  console log data    x000D   x000D   x000D

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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  lt  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

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Simple code with ES6 syntax  return sorted array of duplicates    let duplicates   a   gt   d     a sort  a b    gt  a-b  reduce  a b   gt  a  b amp  amp  d includes a  amp  amp d push a   return b    return d     How to use   duplicates  1 2 3 10 10 2 3 3 10

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This is my proposal  ES6    let a    1  2  3  4  2  2  4  1  5  6  let b       new Set a sort   filter  o  i    gt  o     undefined  amp  amp  a i   1      undefined  amp  amp  o     a i   1         b is now  1  2  4

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program to find the duplicate elements in arraylist  import java util ArrayList  import java util Scanner   public class DistinctEle         public static void main String args                  System out println  Enter elements            ArrayList lt Integer gt  abc new ArrayList lt Integer gt             ArrayList lt Integer gt  ab new ArrayList lt Integer gt             Scanner a new Scanner System in           int b          for int i 0 i lt  10 i                          b a nextInt                if  abc contains b                                 abc add b                             else                               System out println  duplicate elements  b

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function remove dups arrayName     var newArray   new Array       label for var i 0  i lt arrayName length  i              for var j 0  j lt newArray length j             if newArray j   arrayName i             continue label                        newArray newArray length    arrayName i           return newArray

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This is most efficient way i can think of as doesn t include Array indexOf   or Array lastIndexOf   which have complexity of O n  and using inside any loop of complexity O n  will make complete complexity O n 2    My first loop have complexity of O n 2  or O  n 2    1   as complexity of search in hash is O 1   The second loop worst complexity when there s no duplicate in array is O n  and best complexity when every element have a duplicate is O n 2    function duplicates arr      let duplicates             d             i   0        j   arr length - 1        Complexity O n 2    while  i  lt   j        if  i     j        d arr i     d arr i      1   d arr i     1      Complexity O 1      else         d arr i     d arr i      1   d arr i     1      Complexity O 1        d arr j     d arr j      1   d arr j     1      Complexity O 1               i      --j            Worst complexity O n   best complexity O n 2    for  let k in d        if  d k   gt  1        duplicates push k          return duplicates      console log duplicates  5 6 4 9 2 3 5 3 4 1 5 4 9     console log duplicates  2 3 4 5 4 3 4     console log duplicates  4 5 2 9     console log duplicates  4 5 2 9 2 5 9 4

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using Pure Js  function arr      var a   1 2 3 4 5 34 2 5 7 8 6 4 3 25 8 34 56 7 8 76 4 23 34 46 575 8564 53 5345657566     var b        b push a 0      var z 0     for var i 0  i lt  a length  i           for var j 0  j lt  b length  j             if b j     a i              z   0            break                    else           z   1                if z          b push a i            console log b

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This can also be solved using Set     A value in the Set may only occur once  it is unique in the Set s collection       Array prototype hasDuplicates   function              if  arr length      new Set arr  size                 return true                    return false          More information on Sets  https   developer mozilla org en-US docs Web JavaScript Reference Global Objects Set  Note  Sets are not fully supported in IE

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UPDATED  Short one-liner to get the duplicates    1  2  2  4  3  4  filter  e  i  a    gt  a indexOf e      i      2  4    To get the array without duplicates simply invert the condition     1  2  2  4  3  4  filter  e  i  a    gt  a indexOf e      i      1  2  3  4    I simply did not think about filter   in my old answer below       When all you need is to check that there are no duplicates as asked in this question you can use the every   method    1  2  3  every  e  i  a    gt  a indexOf e      i     true   1  2  1  every  e  i  a    gt  a indexOf e      i     false   Note that every   doesn t work for IE 8 and below

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var input     a    b    a    c    c        duplicates           i  j  for  i   0  j   input length  i  lt  j  i        if  duplicates indexOf input i       -1  amp  amp  input indexOf input i   i 1      -1        duplicates push input i           console log duplicates

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Here s one without using a temp Array to store the non-duplicate ones      simple duplicate removal for non-object types Array prototype removeSimpleDupes   function         var i  cntr   0  arr   this  len   arr length       var uniqueVal   function val n len       remove duplicates         var dupe   false              for  i   n  i  lt  len  i                       if  typeof arr i     undefined   amp  amp  val   arr i     arr splice i 1   dupe   true                          return  dupe    arr length   len              while  cntr  lt  len            len   uniqueVal arr cntr  cntr 1 len           cntr               return arr

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This is a single loop approach with a hash table for counting the elements and filter the array if the count is 2  because it returns the first found duplicate   Advantage    single loop uses an object for counting in a closure    x000D   x000D  var array    5  0  2  1  2  3  3  4  4  8  6  7  9  4   x000D      duplicates   array filter  h   gt  v   gt   h v     h v     0    1      2        x000D       x000D  console log duplicates   x000D   x000D   x000D

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To solve the above in O n  time complexity  without sorting    var arr    9  9  111  2  3  4  4  5  7    var obj      for var i 0 i lt arr length i         if  obj arr i             obj arr i   1        else           obj arr i   obj arr i   1          var result    for var key in obj       if obj key  gt 1           result push Number key      change this to result push key  to find duplicate strings in an array          console log result

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Higher ranked answers have a few inherent issues including the use of legacy javascript  incorrect ordering or with only support for 2 duplicated items   Here s a modern solution which fixes those problems   const arrayNonUniq   array   gt        if   Array isArray array             throw new TypeError  An array must be provided               return array filter  value  index    gt  array indexOf value      index  amp  amp  array lastIndexOf value      index     arrayNonUniq  1  1  2  3  3       gt   1  3   arrayNonUniq   foo    foo    bar    foo        gt    foo     You can also use the npm package array-non-uniq

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Here is the one of methods to avoid duplicates into javascript array   and it supports for strings and numbers      var unique   function origArr        var newArray               origLen   origArr length          found          x   0  y   0       for   x   0  x  lt  origLen  x               found   undefined          for   y   0  y  lt  newArray length  y                   if   origArr x      newArray y    found   true                    if    found  newArray push  origArr x                  return newArray      check this fiddle

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You can proceed by comparing index   function getDuplicate array        return array filter  value  index    gt  array value     index

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var a     a   a   b   c   c     a filter function value index self   return  self indexOf value      index

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You can use the following construction   var arr    1 2 3 4 5 6 7 8 9 0 5   var duplicate   arr filter function item  i  arr      return -1     arr indexOf item  i   1

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In this post was useful for duplication check if u are using Jquery   How to find the duplicates in an array using jquery  var unique values       var list of values          input name   recordset          each function item             if     unique values item value                  unique values item value    true              list of values push item value             else                  We have duplicate values

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Just to add some theory to the above    Finding duplicates has a lower bound of O n log n  in the comparison model   SO theoretically  you cannot do any better than first sorting then going through the list sequentially removing any duplicates you find   If you want to find the duplicates in linear  O n    expected time  you could hash each element of the list  if there is a collision  remove label it as a duplicate  and continue

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

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