Simplest way to merge ES6 Maps Sets

Question

Is there a simple way to merge ES6 Maps together  like Object assign   And while we re at it  what about ES6 Sets  like Array concat

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Example const mergedMaps       maps    gt        const dataMap   new Map          for  const map of maps            for  const  key  value  of map                dataMap set key  value                       return dataMap    Usage const map   mergedMaps new Map   1  false     new Map    foo    bar      new Map    lat   1241 173512     Array from map keys        1   foo    lat

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Based off of Asaf Katz s answer  here s a typescript version   export function union lt T gt      iterables  Array lt Set lt T gt  gt    Set lt T gt      const set   new Set lt T gt      iterables forEach iterable   gt        iterable forEach item   gt  set add item          return set

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Edit    I benchmarked my original solution against other solutions suggests here and found that it is very inefficient  The benchmark itself is very interesting  link  It compares 3 solutions  higher is better     fregante  formerly called  bfred it  solution  which adds values one by one  14 955 op sec   jameslk s solution  which uses a self invoking generator  5 089 op sec  my own  which uses reduce  amp  spread  3 434 op sec   As you can see   fregante s solution is definitely the winner  Performance   Immutability With that in mind  here s a slightly modified version which doesn t mutates the original set and excepts a variable number of iterables to combine as arguments  function union    iterables      const set   new Set       for  const iterable of iterables        for  const item of iterable          set add item                return set     Usage  const a   new Set  1  2  3    const b   new Set  1  3  5    const c   new Set  4  5  6     union a b c      1  2  3  4  5  6     Original Answer I would like to suggest another approach  using reduce and the spread operator  Implementation function union  sets      return sets reduce  combined  list    gt        return new Set     combined     list         new Set        Usage  const a   new Set  1  2  3    const b   new Set  1  3  5    const c   new Set  4  5  6     union  a  b  c       1  2  3  4  5  6   Tip  We can also make use of the rest operator to make the interface a bit nicer  function union     sets      return sets reduce  combined  list    gt        return new Set     combined     list         new Set        Now  instead of passing an array of sets  we can pass an arbitrary number of arguments of sets  union a  b  c      1  2  3  4  5  6

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The approved answer is great but that creates a new set every time   If you want to mutate an existing object instead  use a helper function   Set  function concatSets set     iterables        for  const iterable of iterables            for  const item of iterable                set add item                       Usage   const setA   new Set  1  2  3    const setB   new Set  4  5  6    const setC   new Set  7  8  9    concatSets setA  setB  setC      setA will have items 1  2  3  4  5  6  7  8  9   Map  function concatMaps map     iterables        for  const iterable of iterables            for  const item of iterable                map set    item                       Usage   const mapA   new Map   set  S   1  set  P   2   const mapB   new Map   set  Q   3  set  R   4   concatMaps mapA  mapB      mapA will have items   S   1     P   2     Q   3     R   4

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For reasons I do not understand  you cannot directly add the contents of one Set to another with a built-in operation   Operations like union  intersect  merge  etc    are pretty basic set operations  but are not built-in   Fortunately  you can construct these all yourself fairly easily     So  to implement a merge operation  merging the contents of one Set into another or one Map into another   you can do this with a single  forEach   line   var s   new Set  1 2 3    var t   new Set  4 5 6     t forEach s add  s   console log s        1 2 3 4 5 6     And  for a Map  you could do this   var s   new Map    key1   1     key2   2     var t   new Map    key3   3     key4   4      t forEach function value  key        s set key  value         Or  in ES6 syntax   t forEach  value  key    gt  s set key  value        FYI  if you want a simple subclass of the built-in Set object that contains a  merge   method  you can use this      subclass of Set that adds new methods    Except where otherwise noted  arguments to methods      can be a Set  anything derived from it or an Array    Any method that returns a new Set returns whatever class the this object is      allowing SetEx to be subclassed and these methods will return that subclass      For this to work properly  subclasses must not change behavior of SetEx methods       Note that if the contructor for SetEx is passed one or more iterables      it will iterate them and add the individual elements of those iterables to the Set    If you want a Set itself added to the Set  then use the  add   method    which remains unchanged from the original Set object   This way you have    a choice about how you want to add things and can do it either way   class SetEx extends Set          create a new SetEx populated with the contents of one or more iterables     constructor    iterables            super            this merge    iterables                 merge the items from one or more iterables into this set     merge    iterables            for  let iterable of iterables                for  let item of iterable                    this add item                                   return this                        return new SetEx object that is union of all sets passed in with the current set     union    sets            let newSet   new this constructor    sets           newSet merge this           return newSet                return a new SetEx that contains the items that are in both sets     intersect target            let newSet   new this constructor            for  let item of this                if  target has item                     newSet add item                                   return newSet                        return a new SetEx that contains the items that are in this set  but not in target        target must be a Set  or something that supports  has item  such as a Map      diff target            let newSet   new this constructor            for  let item of this                if   target has item                     newSet add item                                   return newSet                        target can be either a Set or an Array        return boolean which indicates if target set contains exactly same elements as this        target elements are iterated and checked for this has item      sameItems target            let tsize          if   size  in target                tsize   target size            else if   length  in target                tsize   target length            else               throw new TypeError  target must be an iterable like a Set with  size or  length                      if  tsize     this size                return false                    for  let item of target                if   this has item                     return false                                  return true           module exports   SetEx    This is meant to be in it s own file setex js that you can then require   into node js and use in place of the built-in Set

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For sets   var merged   new Set     set1     set2     set3     For maps   var merged   new Map     map1     map2     map3     Note that if multiple maps have the same key  the value of the merged map will be the value of the last merging map with that key

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It does not make any sense to call new Set    anArrayOrSet  when adding multiple elements  from either an array or another set  to an existing set   I use this in a reduce function  and it is just plain silly  Even if you have the    array spread operator available  you should not use it in this case  as it wastes processor  memory  and time resources      Add any Map or Set to another function addAll target  source      if  target instanceof Map        Array from source entries    forEach it   gt  target set it 0   it 1        else if  target instanceof Set        source forEach it   gt  target add it           Demo Snippet   x000D   x000D     Add any Map or Set to another x000D  function addAll target  source    x000D    if  target instanceof Map    x000D      Array from source entries    forEach it   gt  target set it 0   it 1    x000D      else if  target instanceof Set    x000D      source forEach it   gt  target add it   x000D      x000D    x000D   x000D  const items1     a    b    c   x000D  const items2     a    b    c    d   x000D  const items3     d    e   x000D   x000D  let set x000D   x000D  set   new Set items1  x000D  addAll set  items2  x000D  addAll set  items3  x000D  console log  adding array to set   Array from set   x000D   x000D  set   new Set items1  x000D  addAll set  new Set items2   x000D  addAll set  new Set items3   x000D  console log  adding set to set   Array from set   x000D   x000D  const map1     x000D      a   1   x000D      b   2   x000D      c   3  x000D    x000D  const map2     x000D      a   1   x000D      b   2   x000D      c   3   x000D      d   4  x000D    x000D  const map3     x000D      d   4   x000D      e   5  x000D    x000D   x000D  const map   new Map map1  x000D  addAll map  new Map map2   x000D  addAll map  new Map map3   x000D  console log  adding map to map   x000D     keys   Array from map keys     x000D     values   Array from map values     x000D   x000D   x000D

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Transform the sets into arrays  flatten them and finally the constructor will uniqify   const union       sets    gt  new Set sets map s   gt      s   flat

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You can use the spread syntax to merge them together   const map1    a  1  b  2  const map2    b  1  c  2  a  5   const mergedMap       a     b     gt   a  5  b  1  c  2

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To merge the sets in the array Sets  you can do  var Sets    set1  set2  set3    var merged   new Set    concat    Sets map set   gt  Array from set        It is slightly mysterious to me why the following  which should be equivalent  fails at least in Babel   var merged   new Set    concat    Sets map Array from

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merge to Map   let merge       map1    map2

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No  there are no builtin operations for these  but you can easily create them your own   Map prototype assign   function    maps        for  const m of maps          for  const kv of m              this add    kv       return this      Set prototype concat   function    sets        const c   this constructor      let res   new  c Symbol species     c         for  const set of  this     sets           for  const v of set              res add v       return res

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Here s my solution using generators   For Maps   let map1   new Map    map2   new Map     map1 set  a    foo    map1 set  b    bar    map2 set  b    baz    map2 set  c    bazz     let map3   new Map function      yield  map1  yield  map2         console log Array from map3       Result       a    foo        b    baz        c    bazz        For Sets   let set1   new Set   foo    bar     set2   new Set   bar    baz      let set3   new Set function      yield  set1  yield  set2         console log Array from set3       Result     foo    bar    baz

[javascript] Simplest way to merge ES6 Maps/Sets?

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