map a Javascript ES6 Map

Question

How would you do this  Instinctively  I want to do   var myMap   new Map    thing1   1     thing2   2     thing3   3         wishful  ignorant thinking var newMap   myMap map  key  value    gt  value   1      Map    thing1    gt  2   thing2    gt  3   thing3    gt  4     I ve haven t gleaned much from the documentation on the new iteration protocol   I am aware of wu js  but I m running a Babel project and don t want to include Traceur  which it seems like it currently depends on   I also am a bit clueless as to how to extract how fitzgen wu js did it into my own project   Would love a clear  concise explanation of what I m missing here  Thanks     Docs for ES6 Map  FYI

User · Answer

You can map   arrays  but there is no such operation for Maps  The solution from Dr  Axel Rauschmayer    Convert the map into an array of  key value  pairs  Map or filter the array  Convert the result back to a map    Example   let map0   new Map      1   a       2   b       3   c        const map1   new Map        map0     map   k  v     gt   k   2        v        resulted in   2   gt    a   4   gt    b   6   gt    c

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Maybe this way  const m   new Map    quot a quot   1     quot b quot   2     quot c quot   3     m map  k  v    gt   k  v   2       Map    a    gt  2   b    gt  4   c    gt  6    You would only need to monkey patch Map before  Map prototype map   function func       return new Map Array from this    k  v     gt  func k  v        We could have wrote a simpler form of this patch  Map prototype map   function func       return new Map Array from this  func       But we would have forced us to then write m map   k  v     gt   k  v   2    which seems a bit more painful and ugly to me  Mapping values only We could also map values only  but I wouldn t advice going for that solution as it is too specific  Nevertheless it can be done and we would have the following API  const m   new Map    quot a quot   1     quot b quot   2     quot c quot   3     m map v   gt  v   2      Map    a    gt  2   b    gt  4   c    gt  6    Just like before patching this way  Map prototype map   function func       return new Map Array from this    k  v     gt   k  func v         Maybe you can have both  naming the second mapValues to make it clear that you are not actually mapping the object as it would probably be expected

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x000D   x000D  const mapMap    callback  map    gt  new Map Array from map  map callback   x000D   x000D  var myMap   new Map    thing1   1     thing2   2     thing3   3     x000D   x000D  var newMap   mapMap  pair    gt   pair 0   pair 1    1   myMap      Map    thing1    gt  2   thing2    gt  3   thing3    gt  4   x000D   x000D   x000D

User · Answer

I prefer to extend the map  export class UtilMap extends Map       constructor    args    super args         public map supplier          const mapped   new UtilMap          this forEach   value  key    gt  mapped set key  supplier value  key             return mapped

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So  map itself only offers one value you care about    That said  there are a few ways of tackling this      instantiation const myMap   new Map        A   1         B   2           what s built into Map for you myMap forEach   val  key    gt  console log key  val         A 1    B 2      what Array can do for you Array from  myMap   map   key  value     gt     key  value           key  A   value  1             less awesome iteration let entries   myMap entries     for  let entry of entries      console log entry       Note  I m using a lot of new stuff in that second example       Array from takes any iterable  any time you d use    slice call     plus Sets and Maps  and turns it into an array        Maps  when coerced into an array  turn into an array of arrays  where el 0      key  amp  amp  el 1      value   basically  in the same format that I prefilled my example Map with  above    I m using destructuring of the array in the argument position of the lambda  to assign those array spots to values  before returning an object for each el   If you re using Babel  in production  you re going to need to use Babel s browser polyfill  which includes  core-js  and Facebook s  regenerator    I m quite certain it contains Array from

User · Answer

You can use myMap forEach  and in each loop  using map set to change value    x000D   x000D  myMap   new Map   x000D      a   1   x000D      b   2   x000D      c   3  x000D      x000D   x000D  for  var  key  value  of myMap entries      x000D    console log key           value   x000D    x000D   x000D   x000D  myMap forEach  value  key  map    gt    x000D    map set key  value 1  x000D     x000D   x000D  for  var  key  value  of myMap entries      x000D    console log key           value   x000D    x000D   x000D   x000D

User · Answer

You should just use Spread operator    x000D   x000D  var myMap   new Map    thing1   1     thing2   2     thing3   3     x000D   x000D  var newArr       myMap  map value   gt  value 1    1   x000D  console log newArr      2  3  4  x000D   x000D  var newArr2    for value of myMap  value   value 1    1   x000D  console log newArr2      2  3  4  x000D   x000D   x000D

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Actually you can still have a Map with the original keys after converting to array with Array from  That s possible by returning an array  where the first item is the key  and the second is the transformed value   const originalMap   new Map       thing1   1     thing2   2     thing3   3       const arrayMap   Array from originalMap    key  value     gt        return  key  value   1      return an array      const alteredMap   new Map arrayMap    console log originalMap      Map    thing1    gt  1   thing2    gt  2   thing3    gt  3   console log alteredMap       Map    thing1    gt  2   thing2    gt  3   thing3    gt  4     If you don t return that key as the first array item  you loose your Map keys

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Using Array from I wrote a Typescript function that maps the values   function mapKeys lt T  V  U gt  m  Map lt T  V gt   fn   this  void  v  V    gt  U   Map lt T  U gt      function transformPair  k  v    T  V     T  U        return  k  fn v         return new Map Array from m entries    transformPair       const m   new Map   1  2    3  4     console log mapKeys m  i   gt  i   1       Map   1   gt  3  3   gt  5

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Map prototype map   function callback      const output   new Map     this forEach  element  key   gt       output set key  callback element  key          return output    const myMap   new Map    thing1   1     thing2   2     thing3   3       no longer wishful thinking const newMap   myMap map  value  key    gt  value   1  console info myMap  newMap    Depends on your religious fervor in avoiding editing prototypes  but  I find this lets me keep it intuitive

User · Answer

Just use Array from iterable   mapFn     var myMap   new Map    thing1   1     thing2   2     thing3   3      var newArr   Array from myMap values    value   gt  value   1

User · Answer

You can use this function   function mapMap map  fn      return new Map Array from map    key  value     gt   key  fn value  key  map          usage   var map1   new Map    A   2     B   3     C   4      var map2   mapMap map1  v   gt  v   v    console log map1  map2      Map   A   2  B   3  C   4   Map   A   4  B   9  C   16

User · Answer

If you don t want to convert the entire Map into an array beforehand  and or destructure key-value arrays  you can use this silly function    x000D   x000D      x000D     Map over an ES6 Map  x000D     x000D      param  Map  map x000D      param  Function  cb Callback  Receives two arguments  key  value  x000D      returns  Array  x000D      x000D  function mapMap map  cb    x000D    let out   new Array map size   x000D    let i   0  x000D    map forEach  val  key    gt    x000D      out i      cb key  val   x000D        x000D    return out  x000D    x000D   x000D  let map   new Map   x000D      a   1   x000D      b   2   x000D      c   3  x000D      x000D   x000D  console log  x000D    mapMap map   k  v    gt     k -  v    join       x000D        a-1  b-2  c-3 x000D   x000D   x000D

[javascript] .map() a Javascript ES6 Map?

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