Getting a union of two arrays in JavaScript

Question

Say I have an array of  34  35  45  48  49  and another array of  48  55   How can I get a resulting array of  34  35  45  48  49  55

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Simple way to deal with merging single array values   var values 0      id  1235  name   value 1   values 1      id  4323  name   value 2    var object null  var first values 0   for  var i in values    if i gt 0       object    merge values i  first

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ES2015 version  Array prototype diff   function a   return this filter i   gt  a indexOf i   lt  0     Array prototype union   function a   return     this diff a      a

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x000D   x000D  function unionArrays     x000D      var args   arguments  x000D      l   args length  x000D      obj       x000D      res       x000D      i  j  k  x000D   x000D      while  l--    x000D          k   args l   x000D          i   k length  x000D   x000D          while  i--    x000D              j   k i   x000D              if   obj j     x000D                  obj j    1  x000D                  res push j   x000D                x000D               x000D        x000D   x000D      return res  x000D    x000D  var unionArr   unionArrays  34  35  45  48  49    44  55    x000D  console log unionArr   x000D   x000D   x000D    Somewhat similar in approach to alejandro s method  but a little shorter and should work with any number of arrays

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function unite arr1  arr2  arr3     newArr arr1 concat arr2  concat arr3     a newArr filter function value      return  arr1 some function value2         return value    value2               console log arr1 concat a        This is for Sorted union following the order

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I like Peter Ajtai s concat-then-unique solution  but the code s not very clear  Here s a nicer alternative   function unique x      return x filter function elem  index    return x indexOf elem      index         function union x  y      return unique x concat y         Since indexOf returns the index of the first occurence  we check this against the current element s index  the second parameter to the filter predicate

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If you don t need to keep the order  and consider 45 and  45  to be the same    x000D   x000D  function union arrays  x  y    x000D    var obj       x000D    for  var i   x length-1  i  gt   0  -- i  x000D       obj x i     x i   x000D    for  var i   y length-1  i  gt   0  -- i  x000D       obj y i     y i   x000D    var res      x000D    for  var k in obj    x000D      if  obj hasOwnProperty k        lt -- optional x000D        res push obj k    x000D      x000D    return res  x000D    x000D   x000D  console log union arrays  34 35 45 48 49    44 55     x000D   x000D   x000D

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If you use the library underscore you can write like this   x000D   x000D  var unionArr     union  34 35 45 48 49    48 55    x000D  console log unionArr   x000D   lt script src  https   cdnjs cloudflare com ajax libs underscore js 1 9 1 underscore-min js  gt  lt  script gt  x000D   x000D   x000D    Ref  http   underscorejs org  union

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If you wants to concatenate two arrays without any duplicate value Just try this  var a  34  35  45  48  49   var b  48  55   var c a concat b  sort    var res c filter  value pos    gt   return c indexOf value     pos

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I would first concatenate the arrays  then I would return only the unique value   You have to create your own function to return unique values  Since it is a useful function  you might as well add it in as a functionality of the Array   In your case with arrays array1 and array2 it would look like this    array1 concat array2  - concatenate the two arrays array1 concat array2  unique   - return only the unique values  Here unique   is a method you added to the prototype for Array    The whole thing would look like this    x000D   x000D  Array prototype unique   function      x000D      var r   new Array    x000D      o  for var i   0  n   this length  i  lt  n  i    x000D        x000D          for var x   0  y   r length  x  lt  y  x    x000D            x000D              if r x   this i   x000D                x000D                  continue o  x000D                x000D            x000D          r r length    this i   x000D        x000D      return r  x000D    x000D  var array1    34 35 45 48 49   x000D  var array2    34 35 45 48 49 55   x000D   x000D     concatenate the arrays then return only the unique values x000D  console log array1 concat array2  unique     x000D   x000D   x000D

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If you want a custom equals function to match your elements  you can use this function in ES2015   function unionEquals left  right  equals       return left concat right  reduce   acc element    gt            return acc some elt   gt  equals elt  element    acc   acc concat element                  It traverses the left right array  Then for each element  will fill the accumulator if it does not find that element in the accumulator  At the end  there are no duplicate as specified by the equals function   Pretty  but probably not very efficient with thousands of objects

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i for  i of new Set array1 concat array2       Let me break this into parts for you     This is a list by comprehension    Store each result in an element of the array  i    will be placed in the variable  i   for each element of        for  i of            the Set which is made of                new Set                        the concatenation of both arrays                 array1 concat array2                          In other words  it first concatenates both and then it removes the duplicates  a Set  by definition cannot have duplicates   Do note  though  that the order of the elements is not guaranteed  in this case

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x000D   x000D  function unionArray arrayA  arrayB    x000D    var obj       x000D        i   arrayA length  x000D        j   arrayB length  x000D        newArray       x000D    while  i--    x000D      if    arrayA i  in obj     x000D        obj arrayA i     true  x000D        newArray push arrayA i    x000D        x000D      x000D    while  j--    x000D      if    arrayB j  in obj     x000D        obj arrayB j     true  x000D        newArray push arrayB j    x000D        x000D      x000D    return newArray  x000D    x000D  var unionArr   unionArray  34  35  45  48  49    44  55    x000D  console log unionArr   x000D   x000D   x000D    Faster  http   jsperf com union-array-faster

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I m probably wasting time on a dead thread here  I just had to implement this and went looking to see if I was wasting my time    I really like KennyTM s answer  That s just how I would attack the problem  Merge the keys into a hash to naturally eliminate duplicates and then extract the keys  If you actually have jQuery you can leverage its goodies to make this a 2 line problem and then roll it into an extension  The each   in jQuery will take care of not iterating over items where hasOwnProperty   is false   jQuery fn extend       union  function array1  array2            var hash       union                 each   merge   merge     array1   array2   function  index  value    hash value    value                each hash  function  key  value    union push key                return union              Note that both of the original arrays are left intact  Then you call it like this   var union     union array1  array2

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Just wrote before for the same reason  works with any amount of arrays           Returns with the union of the given arrays         param Any amount of arrays to be united      returns  array  The union array      function uniteArrays         var union           for  var argumentIndex   0  argumentIndex  lt  arguments length  argumentIndex                  eachArgument   arguments argumentIndex           if  typeof eachArgument      array                         eachArray   eachArgument              for  var index   0  index  lt  eachArray length  index                                  eachValue   eachArray index                   if  arrayHasValue union  eachValue     false                  union push eachValue                                      return union         function arrayHasValue array  value    return array indexOf value     -1

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Shorter version of kennytm s answer    function unionArrays a  b        const cache            a forEach item   gt  cache item    item       b forEach item   gt  cache item    item        return Object keys cache  map key   gt  cache key

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Adapted from  https   stackoverflow com a 4026828 1830259  Array prototype union   function a         var r   this slice 0       a forEach function i    if  r indexOf i   lt  0  r push i           return r      Array prototype diff   function a        return this filter function i   return a indexOf i   lt  0         var s1    1  2  3  4   var s2    3  4  5  6    console log  s1      s1   console log  s2      s2   console log  s1 union s2       s1 union s2    console log  s2 union s1       s2 union s1    console log  s1 diff s2       s1 diff s2    console log  s2 diff s1       s2 diff s1        Output     s1  1 2 3 4    s2  3 4 5 6    s1 union s2   1 2 3 4 5 6    s2 union s1   3 4 5 6 1 2    s1 diff s2   1 2    s2 diff s1   5 6

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You can try these   function union a  b        return a concat b  reduce function prev  cur            if  prev indexOf cur      -1  prev push cur           return prev                      or  function union a  b        return a concat b filter function el            return a indexOf el      -1

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I think it would be simplest to create a new array  adding the unique values only as determined by indexOf   This seems to me to be the most straightforward solution  though I don t know if it is the most efficient  Collation is not preserved   var a    34  35  45  48  49       b    48  55    var c   union a  b    function union a  b       will work for n  gt   2 inputs     var newArray              cycle through input arrays     for  var i   0  l   arguments length  i  lt  l  i                 cycle through each input arrays elements         var array   arguments i           for  var ii   0  ll   array length  ii  lt  ll  ii                  var val   array ii                  only add elements to the new array if they are unique             if  newArray indexOf val   lt  0  newArray push val                       return newArray

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x000D   x000D  function unique arrayName  x000D    x000D    var newArray new Array    x000D    label  for var i 0  i lt arrayName length i     x000D        x000D      for var j 0  j lt newArray length j     x000D        x000D        if newArray j   arrayName i    x000D          continue label  x000D        x000D      newArray newArray length    arrayName i   x000D      x000D    return newArray  x000D    x000D   x000D  var arr1   new Array 0 2 4 4 4 4 4 5 5 6 6 6 7 7 8 9 5 1 2 3 0   x000D  var arr2  new Array 3 5 8 1 2 32 1 2 1 2 4 7 8 9 1 2 1 2 3 4 5   x000D  var union   unique arr1 concat arr2    x000D  console log union   x000D   x000D   x000D

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You can use a jQuery plugin  jQuery Array Utilities  For example the code below     union  1  2  2  3    2  3  4  5  5     will return  1 2 3 4 5

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With the arrival of ES6 with sets and splat operator  at the time of being works only in Firefox  check compatibility table   you can write the following cryptic one liner    x000D   x000D  var a    34  35  45  48  49   x000D  var b    48  55   x000D  var union       new Set     a     b     x000D  console log union   x000D   x000D   x000D    Little explanation about this line      a     b  concatenates two arrays  you can use a concat b  as well  new Set   create a set out of it and thus your union  And the last     x  converts it back to an array

[javascript] Getting a union of two arrays in JavaScript

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