map function for objects instead of arrays

Question

I have an object   myObject      a   1   b   2   c   3     I am looking for a native method  similar to Array prototype map that would be used as follows   newObject   myObject map function  value  label        return value   value          newObject is now    a   1   b   4   c   9     Does JavaScript have such a map function for objects   I want this for Node JS  so I don t care about cross-browser issues

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x000D   x000D  const orig      a   1   b   2   c   3   x000D   x000D  const result     transform orig   r  v  k    gt  r k trim      v   2   x000D   x000D  console log result   x000D   lt script src  https   cdnjs cloudflare com ajax libs lodash js 4 17 4 lodash min js  gt  lt  script gt  x000D   x000D   x000D    Use new    transform   to transforms object

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If you re interested in mapping not only values but also keys  I have written Object map valueMapper  keyMapper   that behaves this way   var source     a  1  b  2    function sum x    return x   x    source map sum                 returns   a  2  b  4   source map undefined  sum      returns   aa  1  bb  2   source map sum  sum            returns   aa  2  bb  4

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var myObject      a   1   b   2   c   3      Object prototype map   function fn       var oReturn           for  sCurObjectPropertyName in this            oReturn sCurObjectPropertyName    fn this sCurObjectPropertyName   sCurObjectPropertyName             return oReturn    Object defineProperty Object prototype  map   enumerable false         newObject   myObject map function  value  label        return value   value           newObject is now    a   1   b   4   c   9

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JavaScript just got the new Object fromEntries method   Example   x000D   x000D  function mapObject  obj  fn    x000D    return Object fromEntries  x000D      Object x000D         entries obj  x000D         map fn  x000D      x000D    x000D   x000D  const myObject     a  1  b  2  c  3   x000D  const myNewObject   mapObject myObject    key  value     gt    key  value   value    x000D  console log myNewObject  x000D   x000D   x000D    Explanation  The code above converts the Object into an nested Array     lt key gt   lt value gt          wich you can map over  Object fromEntries converts the Array back to an Object   The cool thing about this pattern  is that you can now easily take object keys into account while mapping   Documentation   Object fromEntries   Object entries     Browser Support  Object fromEntries is currently only supported by these browsers engines  nevertheless there are polyfills available  e g  babel polyfill

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I handle only strings to reduce exemptions    Object keys params  map k   gt  typeof params k      string    params k    params k  trim     null

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Hey wrote a little mapper function that might help       function propertyMapper object  src            for  var property in object                  for  var sourceProp in src                   if property     sourceProp                    if Object prototype toString call  property         object Array                        propertyMapper object property   src sourceProp                        else                     object property    src sourceProp

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x000D   x000D  var myObject      a   1   b   2   c   3    x000D  Object keys myObject  filter  item    gt  myObject item     2  x000D  console log myObject  x000D   x000D   x000D

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I specifically wanted to use the same function that I was using for arrays for a single object  and wanted to keep it simple  This worked for me   var mapped    item  map myMapFunction  pop

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The accepted answer has two drawbacks    It misuses Array prototype reduce  because reducing means to change the structure of a composite type  which doesn t happen in this case  It is not particularly reusable     An ES6 ES2015 functional approach  Please note that all functions are defined in curried form    x000D   x000D     small  reusable auxiliary functions x000D   x000D  const keys   o   gt  Object keys o   x000D   x000D  const assign       o    gt  Object assign        o   x000D   x000D  const map   f   gt  xs   gt  xs map x   gt  f x    x000D   x000D  const mul   y   gt  x   gt  x   y  x000D   x000D  const sqr   x   gt  mul x   x   x000D   x000D   x000D     the actual map function x000D   x000D  const omap   f   gt  o   gt    x000D    o   assign o      A x000D    map x   gt  o x    f o x     keys o       B x000D    return o  x000D     x000D   x000D   x000D     mock data x000D   x000D  const o     a  1   b  2   c  3   x000D   x000D   x000D     and run x000D   x000D  console log omap sqr   o    x000D  console log omap mul 10    o    x000D   x000D   x000D     In line A o is reassigned  Since Javascript passes reference values by sharing  a shallow copy of o is generated  We are now able to mutate o within omap without mutating o in the parent scope  In line B map s return value is ignored  because map performs a mutation of o  Since this side effect remains within omap and isn t visible in the parent scope  it is totally acceptable    This is not the fastest solution  but a declarative and reusable one  Here is the same implementation as a one-line  succinct but less readable   const omap   f   gt  o   gt   o   assign o   map x   gt  o x    f o x     keys o    o       Addendum - why are objects not iterable by default   ES2015 specified the iterator and iterable protocols  But objects are still not iterable and thus not mappable  The reason is the mixing of data and program level

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The map function does not exist on the Object prototype however you can emulate it like so  var myMap   function   obj  callback          var result            for   var key in obj             if   Object prototype hasOwnProperty call  obj  key                   if   typeof callback      function                      result  key     callback call  obj  obj  key    key  obj                                       return result       var myObject      a   1   b   2   c   3     var newObject   myMap  myObject  function   value  key         return value   value

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Based on  Amberlamps answer  here s a utility function  as a comment it looked ugly   function mapObject obj  mapFunc       return Object keys obj  reduce function newObj  value            newObj value    mapFunc obj value            return newObj                  and the use is   var obj    a 1  b 3  c 5  function double x  return x   2   var newObj   mapObject obj  double       gt    a  2  b  6  c  10

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I came here looking to find and answer for mapping an object to an array and got this page as a result  In case you came here looking for the same answer I was  here is how you can map and object to an array    You can use map to return a new array from the object like so   var newObject   Object keys myObject  map function key       return myObject key

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Define a function mapEntries   mapEntries takes a callback function that is called on each entry in the object with the parameters value  key and object  It should return the a new value   mapEntries should return a new object with the new values returned from the callback   Object defineProperty Object prototype   mapEntries       enumerable  false    value  function  mapEntriesCallback        return Object fromEntries        Object entries this  map            key  value     gt   key  mapEntriesCallback value  key  this                             Usage example   var object    a  1  b  2  c  3  var newObject   object mapEntries value   gt  value   value  console log newObject     gt   a  1  b  4  c  9    Edit  A previous version didn t specify that this is not an enumerable property

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Use following map function to define myObject map o   gt  f  gt  Object keys o  reduce  a c   gt  c   map    a    a c  f o c  c  a         x000D   x000D  let map   o   gt  f  gt  Object keys o  reduce  a c   gt  c   map    a    a c  f o c  c  a             TEST init  myObject      a   1   b   2   c   3   myObject map   map myObject               you can do this instead above line but it is not recommended       you will see  map  key any all objects     Object prototype map   map myObject       OP desired interface described in question newObject   myObject map function  value  label        return  value   value       console log newObject   x000D   x000D   x000D

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const mapObject    targetObject  callbackFn    gt        if   targetObject  return targetObject      if  Array isArray targetObject            return targetObject map  v   gt mapObject v  callbackFn             return Object entries targetObject  reduce  acc  key  value     gt            const res   callbackFn key  value           if   Array isArray res   amp  amp  typeof res     object                return     acc   key   mapObject res  callbackFn                     if  Array isArray res                return     acc   key   res map  v   gt mapObject v  callbackFn                      return     acc   key   res                const mapped   mapObject a  key value   gt        if   Array isArray value   amp  amp  key      a   return       if   Array isArray value   amp  amp  key      e   return         if   Array isArray value   amp  amp  key      g   return value   value      return value      console log JSON stringify mapped          b  2  c     d  2  e      f     g  4        This function goes recursively through the object and arrays of objects  Attributes can be deleted if returned undefined

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I needed a version that allowed modifying the keys as well  based on  Amberlamps and  yonatanmn answers    var facts        can be an object or array - see jsfiddle below      uuid  asdfasdf  color  red         uuid  sdfgsdfg  color  green         uuid  dfghdfgh  color  blue       var factObject   mapObject     facts  function key  item        return  item uuid   test item color  oldKey key         function mapObject empty  obj  mapFunc       return Object keys obj  reduce function newObj  key            var kvPair   mapFunc key  obj key            newObj kvPair 0     kvPair 1           return newObj         empty       factObject      asdfasdf     color   red   oldKey   0     sdfgsdfg     color   green   oldKey   1     dfghdfgh     color   blue   oldKey   2       Edit  slight change to pass in the starting object      Allows it to be     if the keys are integers

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A different take on it is to use a custom json stringify function that can also work on deep objects  This might be useful if you intend to post it to the server anyway as json   x000D   x000D  const obj      a   1   b   2  x    c   3    x000D  const json   JSON stringify obj   k  v    gt  typeof v      number    v   v   v  x000D   x000D  console log json  x000D  console log  back to json    JSON parse json   x000D   x000D   x000D

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I came upon this as a first-item in a Google search trying to learn to do this  and thought I would share for other folsk finding this recently the solution I found  which uses the npm package immutable   I think its interesting to share because immutable uses the OP s EXACT situation in their own documentation - the following is not my own code but pulled from the current immutable-js documentation   const   Seq     require  immutable   const myObject     a  1  b  2  c  3   Seq myObject  map x   gt  x   x  toObject         a  1  b  4  c  9      Not that Seq has other properties   Seq describes a lazy operation  allowing them to efficiently chain use of all the higher-order collection methods  such as map and filter  by not creating intermediate collections   and that some other immutable-js data structures might also do the job quite efficiently   Anyone using this method will of course have to npm install immutable and might want to read the docs   https   facebook github io immutable-js

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2020 updates  overwriting Object prototype  Object prototype map   function func       for const  k  v  of Object entries this            func k  v             return this     Object prototype reject   function func       for const  k  v  of Object entries this            if func k  v                delete this k                       return this     Object prototype filter   function func       let res           for const  k  v  of Object entries this            if  func k  v                delete this k                       return this     a         1    a    1        2    b    2        3    a    3        console log Object assign     a  map  k  v    gt  v push  nice                 1      a    1    nice           2      b    2    nice           3      a    3    nice          console log Object assign     a  reject  k  v   gt v 0       a           2      b    2    nice       console log Object assign     a  filter  k  v   gt v 0       a           1      a    1    nice      3      a    3    nice

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ES6   Object prototype map   function mapFunc        return Object keys this  map  key  index    gt  mapFunc key  this key   index        ES2015   Object prototype map   function  mapFunc        var  this   this       return Object keys this  map function  key  index            return mapFunc key   this key   index                test in node    gt  a    foo   bar     foo   bar     gt  a map  k v i    gt  v     bar

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x000D   x000D  var myObject      a   1   b   2   c   3     for  var key in myObject      if  myObject hasOwnProperty key         myObject key     2         console log myObject         a   2   b   4   c   6   x000D   x000D   x000D

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No native methods  but lodash mapValues will do the job brilliantly    mapValues    a   1   b   2   c   3    function num    return num   3              a   3   b   6   c   9

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There is no native map to the Object object  but how about this    x000D   x000D  var myObject      a   1   b   2   c   3    x000D   x000D  Object keys myObject  map function key  index    x000D    myObject key     2  x000D      x000D   x000D  console log myObject   x000D       gt     a   2   b   4   c   6   x000D   x000D   x000D    But you could easily iterate over an object using for     in    x000D   x000D  var myObject      a   1   b   2   c   3    x000D   x000D  for  var key in myObject    x000D    if  myObject hasOwnProperty key     x000D      myObject key     2  x000D      x000D    x000D   x000D  console log myObject   x000D        a   2   b   4   c   6   x000D   x000D   x000D    Update  A lot of people are mentioning that the previous methods do not return a new object  but rather operate on the object itself  For that matter I wanted to add another solution that returns a new object and leaves the original object as it is    x000D   x000D  var myObject      a   1   b   2   c   3    x000D   x000D     returns a new object with the values at each key mapped using mapFn value  x000D  function objectMap object  mapFn    x000D    return Object keys object  reduce function result  key    x000D      result key    mapFn object key   x000D      return result x000D           x000D    x000D   x000D  var newObject   objectMap myObject  function value    x000D    return value   2 x000D     x000D   x000D  console log newObject   x000D       gt     a   2   b   4   c   6   x000D   x000D  console log myObject   x000D       gt     a   1   b   2   c   3   x000D   x000D   x000D    Array prototype reduce reduces an array to a single value by somewhat merging the previous value with the current  The chain is initialized by an empty object     On every iteration a new key of myObject is added with twice the key as the value   Update  With new ES6 features  there is a more elegant way to express objectMap    x000D   x000D  const objectMap    obj  fn    gt  x000D    Object fromEntries  x000D      Object entries obj  map  x000D          k  v   i    gt   k  fn v  k  i   x000D        x000D      x000D     x000D  const myObject     a  1  b  2  c  3   x000D   x000D  console log objectMap myObject  v   gt  2   v    x000D   x000D   x000D

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How about a one-liner in JS ES10   ES2019   Making use of Object entries   and Object fromEntries    let newObj   Object fromEntries Object entries obj  map   k  v     gt   k  v   v      The same thing written as a function  function objMap obj  func      return Object fromEntries Object entries obj  map   k  v     gt   k  func v            To square each value you can call it like this  let mappedObj   objMap obj   x    gt  x   x    This function uses recursion to square nested objects as well  function objMap obj  func      return Object fromEntries      Object entries obj  map   k  v     gt          k  v     Object v    objMap v  func    func v                    To square each value you can call it like this  let mappedObj   objMap obj   x    gt  x   x    With ES7   ES2016 you cant  use Objects fromEntries  but you can achieve the same using Object assign in combination with spread operators and computed key names syntax  let newObj   Object assign    Object entries obj  map   k  v     gt     k   v   v       ES6   ES2015 Doesn t allow Object entires  but you could use Object keys instead  let newObj   Object assign    Object keys obj  map k   gt     k   obj k    obj k        ES6 also introduced for   of loops  which allow a more imperative style  let newObj       for  let  k  v  of Object entries obj       newObj k    v   v       array reduce   Instead of Object fromEntries and Object assign you can also use reduce for this  let newObj   Object entries obj  reduce  p   k  v     gt        p   k   v   v             Inherited properties and the prototype chain  In some rare situation you may need to map a class-like object which holds properties of an inherited object on its prototype-chain  In such cases Object keys   and Object entries   won t work  because these functions do not include the prototype chain  If you need to map inherited properties  you can use for  key in myObj         Here is an example of such situation  const obj1      a   1   b   2   c   3  const obj2   Object create obj1       One of multiple ways to inherit an object in JS      Here you see how the properties of obj1 sit on the  prototype  of obj2 console log obj2      Prints  obj2   proto        a   1   b   2   c   3   console log Object keys obj2        Prints  an empty Array  console log Object entries obj2        Prints  an empty Array   for  let key in obj2      console log key                   Prints   a    b    c     However  please do me a favor and avoid inheritance   -

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It s pretty easy to write one   Object map   function o  f  ctx        ctx   ctx    this      var result           Object keys o  forEach function k            result k    f call ctx  o k   k  o                return result      with example code    gt  o     a  1  b  2  c  3     gt  r   Object map o  function v  k  o         return v   v         gt  r   a   1  b  4  c  9     NB  this version also allows you to  optionally  set the this context for the callback  just like the Array method   EDIT - changed to remove use of Object prototype  to ensure that it doesn t clash with any existing property named map on the object

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My response is largely based off the highest rated response here and hopefully everyone understands  have the same explanation on my GitHub  too   This is why his impementation with map works   Object keys images  map  key    gt  images key     url           images key                      The purpose of the function is to take an object and modify the original contents of the object using a method available to all objects  objects and arrays alike  without returning an array  Almost everything within JS is an object  and for that reason elements further down the pipeline of inheritance can potentially technically use those available to those up the line  and the reverse it appears    The reason that this works is due to the  map functions returning an array REQUIRING that you provide an explicit or implicit RETURN of an array instead of simply modifying an existing object  You essentially trick the program into thinking the object is an array by using Object keys which will allow you to use the map function with its acting on the values the individual keys are associated with  I actually accidentally returned arrays but fixed it   As long as there isn t a return in the normal sense  there will be no array created with the original object stil intact and modified as programmed   This particular program takes an object called images and takes the values of its keys and appends url tags for use within another function  Original is this   var images      snow   https   www trbimg com img-5aa059f5 turbine bs-md-weather-20180305    sunny   http   www cubaweather org images weather-photos large Sunny-morning-east-    Matanzas-city- Cuba-20170131-1080 jpg    rain   https   i pinimg com originals 23 d8  ab 23d8ab1eebc72a123cebc80ce32b43d8 jpg          and modified is this   var images      snow  url  https   www trbimg com img-5aa059f5 turbine bs-md-weather-20180305         sunny  url  http   www cubaweather org images weather-photos large Sunny-morning-    east-Matanzas-city- Cuba-20170131-1080 jpg     rain  url  https   i pinimg com originals 23 d8  ab 23d8ab1eebc72a123cebc80ce32b43d8 jpg         The object s original structure is left intact allowing for normal property access as long as there isn t a return  Do NOT have it return an array like normal and everything will be fine  The goal is REASSIGNING the original values  images key   to what is wanted and not anything else  As far as I know  in order to prevent array output there HAS to be REASSIGNMENT of images key  and no implicit or explicit request to return an array  variable assignment does this and was glitching back and forth for me    EDIT   Going to address his other method regarding new object creation to avoid modifying original object  and reassignment appears to still be necessary in order to avoid accidentally creating an array as output   These functions use arrow syntax and are if you simply want to create a new object for future use   const mapper    obj  mapFn    gt  Object keys obj  reduce  result  key    gt                    result key    mapFn obj  key                   return result                       var newImages   mapper images   value    gt  value     The way these functions work is like so    mapFn takes the function to be added later  in this case  value     value  and simply returns whatever is stored there as a value for that key  or multiplied by two if you change the return value like he did  in mapFn obj  key     and then redefines the original value associated with the key in result key    mapFn obj  key    and returns the operation performed on result  the accumulator located in the brackets initiated at the end of the  reduce function    All of this is being performed on the chosen object and STILL there CANNOT be an implicit request for a returned array and only works when reassigning values as far as I can tell  This requires some mental gymnastics but reduces the lines of code needed as can be seen above  Output is exactly the same as can be seen below    snow   https   www trbimg com img-5aa059f5 turbine bs-    md-weather-20180305   sunny   http   www cubaweather org images weather- photos l   morning-east-Matanzas-city-Cuba-20170131-1080 jpg   rain    https   i pinimg com originals 23 d8  ab 23d8ab1eebc72a123cebc80ce32b43d8 jpg     Keep in mind this worked with NON-NUMBERS  You CAN duplicate ANY object by SIMPLY RETURNING THE VALUE in the mapFN function

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To responds more closely to what precisely the OP asked for  the OP wants an object      myObject      a   1   b   2   c   3     to have a map method myObject map      similar to Array prototype map that would be used as follows   newObject   myObject map function  value  label        return value   value         newObject is now    a   1   b   4   c   9     The imho best  measured in terms to  close to what is asked     no ES 5 6 7  required needlessly   answer would be   myObject map   function mapForObject callback      var result         for var property in this       if this hasOwnProperty property   amp  amp  property     map          result property    callback this property  property this               return result      The code above avoids intentionally using any language features  only available in recent ECMAScript editions  With the code above the problem can be solved lke this     x000D   x000D  myObject      a   1   b   2   c   3    x000D   x000D  myObject map   function mapForObject callback  x000D    x000D    var result       x000D    for var property in this   x000D      if this hasOwnProperty property   amp  amp  property     map    x000D        result property    callback this property  property this   x000D        x000D      x000D    return result  x000D    x000D   x000D  newObject   myObject map function  value  label    x000D    return value   value  x000D      x000D  console log  newObject is now  newObject   x000D   x000D   x000D   alternative test code here  Besides frowned upon by some  it would be a possibility to insert the solution in the prototype chain like this   Object prototype map   function callback      var result         for var property in this       if this hasOwnProperty property          result property    callback this property  property this               return result      Something  which when done with careful oversight should not have any ill effects and not impact map method of other objects  i e  Array s map

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For maximum performance   If your object doesn t change often but needs to be iterated on often I suggest using a native Map as a cache    x000D   x000D     example object x000D  var obj    a  1  b  2  c   something    x000D   x000D     caching map x000D  var objMap   new Map Object entries obj    x000D   x000D     fast iteration on Map object x000D  objMap forEach  item  key    gt    x000D       do something with an item x000D    console log key  item   x000D      x000D   x000D   x000D    Object entries already works in Chrome  Edge  Firefox and beta Opera so it s a future-proof feature  It s from ES7 so polyfill it https   github com es-shims Object entries for IE where it doesn t work

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You can convert an object to array simply by using the following  You can convert the object values to an array   x000D   x000D  myObject      a   1   b   2   c   3     let valuesArray   Object values myObject    console log valuesArray   x000D   x000D   x000D   You can convert the object keys to an array   x000D   x000D  myObject      a   1   b   2   c   3     let keysArray   Object keys myObject    console log keysArray   x000D   x000D   x000D   Now you can perform normal array operations  including the  map  function

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settings       message notification        value  true      is active  true      slug   message notification       title   Message Notification         support notification        value  true      is active  true      slug   support notification       title   Support Notification           let keys   Object keys settings   keys map key  gt  settings key  value   false   console log settings

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If anyone was looking for a simple solution that maps an object to a new object or to an array      Maps an object to a new object by applying a function to each key value pair     Takes the object to map and a function from  key  value  to mapped value  const mapObject    obj  fn    gt        const newObj           Object keys obj  forEach k   gt    newObj k    fn k  obj k            return newObj         Maps an object to a new array by applying a function to each key value pair     Takes the object to map and a function from  key  value  to mapped value  const mapObjectToArray    obj  fn    gt        Object keys obj  map k   gt  fn k  obj k         This may not work for all objects or all mapping functions  but it works for plain shallow objects and straightforward mapping functions which is all I needed

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Minimal version  es6    Object entries obj  reduce  a   k  v     gt   a k    v   v  a

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This is really annoying  and everyone in the JS community knows it  There should be this functionality  const obj1    a 4  b 7   const obj2   Object map obj1   k v    gt  v   5    console log obj1       a 4  b 7  console log obj2       a 9  b 12   here is the na  ve implementation  Object map   function obj  fn  ctx        const ret            for let k of Object keys obj            ret k    fn call ctx    null  k  obj k                 return ret      it is super annoying to have to implement this yourself all the time    If you want something a little more sophisticated  that doesn t interfere with the Object class  try this  let map   function  obj  fn  ctx      return Object keys obj  reduce  a  b    gt        a b    fn call ctx    null  b  obj b        return a                 const x   map  a  2  b  4    k v    gt        return v 2       but it is safe to add this map function to Object  just don t add to Object prototype  Object map          fairly safe Object prototype map        not ok

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First  convert your HTMLCollection using Object entries collection   Then it   s an iterable you can now use the  map method on it   Object entries collection  map        reference  https   medium com  js tut calling-javascript-code-on-multiple-div-elements-without-the-id-attribute-97ff6a50f31

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EDIT  The canonical way using newer JavaScript features is -  const identity   x   gt    x  const omap    f   identity  o         gt    Object fromEntries      Object entries o  map    k  v      gt          k  f v                Where o is some object and f is your mapping function  Or we could say  given a function from a - gt  b  and an object with values of type a  produce an object with values of type b  As a pseudo type signature -     omap    a - gt  b    a    - gt    b       The original answer was written to demonstrate a powerful combinator  mapReduce which allows us to think of our transformation in a different way   m  the mapping function     gives you a chance to transform the incoming element before hellip  r  the reducing function     this function combines the accumulator with the result of the mapped element   Intuitively  mapReduce creates a new reducer we can plug directly into Array prototype reduce  But more importantly  we can implement our object functor implementation omap plainly by utilizing the object monoid  Object assign and       x000D   x000D  const identity   x   gt  x000D    x x000D     x000D  const mapReduce    m  r    gt  x000D     a  x    gt  r  a  m  x   x000D   x000D  const omap    f   identity  o         gt  x000D    Object x000D       keys  o  x000D       reduce x000D          mapReduce x000D              k   gt      k   f  o k      x000D              Object assign x000D              x000D             x000D          x000D             x000D  const square   x   gt  x000D    x   x x000D     x000D  const data   x000D      a   1  b   2  c   3   x000D     x000D  console  log  omap  square  data   x000D       a   1  b   4  c   9   x000D   x000D   x000D    Notice the only part of the program we actually had to write is the mapping implementation itself       k   gt      k   f  o k        Which says  given a known object o and some key k  construct an object and whose computed property k is the result of calling f on the key s value  o k    We get a glimpse of mapReduce s sequencing potential if we first abstract oreduce     oreduce    string   a - gt  string   b  b    a    - gt    b   const oreduce    f   identity  r   null  o         gt    Object      keys  o       reduce         mapReduce             k   gt    k  o k                f                     r             omap    a - gt  b   a   - gt   b  const omap    f   identity  o         gt    oreduce       mapReduce              k  v      gt                  k   f  v               Object assign                          o         Everything works the same  but omap can be defined at a higher-level now  Of course the new Object entries makes this look silly  but the exercise is still important to the learner   You won t see the full potential of mapReduce here  but I share this answer because it s interesting to see just how many places it can be applied  If you re interested in how it is derived and other ways it could be useful  please see this answer

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you can use map method and forEach on arrays but if you want to use it on Object then you can use it with twist like below   Using Javascript  ES6   var obj      a   2   b   4   c   6       Object entries obj  map  v   gt  obj v 0      v 1     console log obj     it will log as  a  4  b  16  c  36   var obj2      a   4   b   8   c   10    Object entries obj2  forEach  v   gt  obj2 v 0      v 1     console log obj2     it will log as  a  16  b  64  c  100    Using jQuery  var ob      a   2   b   4   c   6      map ob  function  val  key       ob key     val      console log ob    it will log as  a  4  b  16  c  36    Or you can use other loops also like   each method as below example     each ob function  key  value      ob key     value      console log ob    it will also log as  a  4  b  16  c  36

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You could use Object keys and then forEach over the returned array of keys   var myObject      a   1   b   2   c   3        newObject       Object keys myObject  forEach function  key        var value   myObject key       newObject key    value   value        Or in a more modular fashion   function map obj  callback        var result           Object keys obj  forEach function  key            result key    callback call obj  obj key   key  obj               return result     newObject   map myObject  function x    return x   x        Note that Object keys returns an array containing only the object s own enumerable properties  thus it behaves like a for  in loop with a hasOwnProperty check

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Object Mapper in TypeScript  I like the examples that use Object fromEntries such as this one  but still  they are not very easy to use  The answers that use Object keys and then look up the key are actually doing multiple look-ups that may not be necessary   I wished there was an Object map function  but we can create our own and call it objectMap with the ability to modify both key and value   Usage  JavaScript    const myObject      a   1   b   2   c   3        keep the key and modify the value let obj   objectMap myObject  val   gt  val   2      obj     a  2  b  4  c  6        modify both key and value obj   objectMap myObject      val   gt  val   2           key   gt   key   key  toUpperCase        obj     AA   2   BB   4   CC   6      Code  TypeScript    interface Dictionary lt T gt         key  string   T     function objectMap lt TValue  TResult gt       obj  Dictionary lt TValue gt       valSelector   val  TValue  obj  Dictionary lt TValue gt     gt  TResult      keySelector    key  string  obj  Dictionary lt TValue gt     gt  string      ctx   Dictionary lt TValue gt          const ret      as Dictionary lt TResult gt       for  const key of Object keys obj             const retKey   keySelector               keySelector call ctx    null  key  obj                key          const retVal   valSelector call ctx    null  obj key   obj           ret retKey    retVal            return ret      If you are not using TypeScript then copy the above code in TypeScript Playground to get the JavaScript code   Also  the reason I put keySelector after valSelector in the parameter list  is because it is optional     Some credit go to alexander-mills  answer

[javascript] map function for objects (instead of arrays)

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