Map over object preserving keys

Question

The map function in underscore js  if called with a javascript object  returns an array of values mapped from the object s values     map  one  1  two  2  three  3   function num  key   return num   3        gt   3  6  9    is there a way to make it preserve the keys  ie  I want a function that returns   one  3  two  6  three  9

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const mapObject    obj       mapper    gt    Object entries obj  reduce       acc   key  val     gt        acc   key   mapper val

User · Answer

map returns an Array  not an Object   If you want an object you re better off using a different function  like each  if you really want to use map you could do something like this   Object keys object  map function value  index       object value     3       but that is confusing  when seeing map one would expect to have an array as result and then make something with it

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A mix fix for the underscore map bug  P    mixin        mapobj   function  obj  iteratee  context             if  obj    null  return             iteratee     iteratee iteratee  context           var keys   obj length      obj length  amp  amp    keys obj               length    keys    obj  length              results                   currentKey          for  var index   0  index  lt  length  index                currentKey   keys   keys index    index            results currentKey    iteratee obj currentKey   currentKey  obj                     if     isObject  obj                   return   object  results                        return results               A simple workaround that keeps the right key and return as object  It is still used the same way as i guest you could used this function  to override the bugy   map function   or simply as me used it as a mixin     mapobj   options   function  val  key  list

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x000D   x000D  var mapped     reduce   one  1  two  2  three  3    function obj  val  key    x000D      obj key    val 3  x000D      return obj  x000D          x000D   x000D  console log mapped   x000D   lt script src  http   underscorejs org underscore-min js  gt  lt  script gt  x000D   lt script src  https   getfirebug com firebug-lite-debug js  gt  lt  script gt  x000D   x000D   x000D

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I know it s been a long time  but still the most obvious solution via fold  aka reduce in js  is missing  for the sake of completeness i ll leave it here   function mapO f  o      return Object keys o  reduce  acc  key    gt        acc key    f o key       return acc

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map using lodash  like loop to achieve this  var result       map  one  1  two  2  three  3   function num  key   result key  num   3      console log result     output  one  1  two  2  three  3   Reduce is clever looks like above answare   reduce  one  1  two  2  three  3   function result  num  key      result key  num   3   return result             output  one  1  two  2  three  3

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With Underscore  Underscore provides a function   mapObject to map the values and preserve the keys     mapObject   one  1  two  2  three  3    function  v    return v   3            gt    one  3  two  6  three  9     DEMO    With Lodash  Lodash provides a function   mapValues to map the values and preserve the keys     mapValues   one  1  two  2  three  3    function  v    return v   3            gt    one  3  two  6  three  9     DEMO

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I know this is old  but now Underscore has a new map for objects      mapObject object  iteratee   context      You can of course build a flexible map for both arrays and objects    fmap   function arrayOrObject  fn  context       if this isArray arrayOrObject         return   map arrayOrObject  fn  context       else       return   mapObject arrayOrObject  fn  context

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I think you want a mapValues function  to map a function over the values of an object   which is easy enough to implement yourself   mapValues   function obj  f      var k  result  v    result         for  k in obj        v   obj k       result k    f v         return result

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You can use   mapValues users  function o    return o age      in Lodash and   mapObject   one  1  two  2  three  3    function  v    return v   3      in Underscore   Check out the cross-documentation here  http   jonathanpchen com underdash-api  mapvalues-object-iteratee-identity

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I managed to find the required function in lodash  a utility library similar to underscore   http   lodash com docs mapValues     mapValues object   callback identity    thisArg         Creates an object with the same keys as object and values generated by    running each own enumerable property of object through the callback     The callback is bound to thisArg and invoked with three arguments     value  key  object

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How about this version in plain JS  ES6   ES2015    let newObj   Object assign    Object keys obj  map k   gt     k   obj k    3        jsbin  If you want to map over an object recursively  map nested obj   it can be done like this   const mapObjRecursive    obj    gt      Object keys obj  forEach key   gt        if  typeof obj key       object   obj key    mapObjRecursive obj key        else obj key    obj key    3          return obj       jsbin  Since ES7   ES2016 you can use Object entries instead of Object keys like this   let newObj   Object assign    Object entries obj  map  k  v    gt     k   v   3

[javascript] Map over object preserving keys

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