Javascript reduce on Object

Question

There is nice Array method reduce   to get one value from the Array  Example    0 1 2 3 4  reduce function previousValue  currentValue  index  array     return previousValue   currentValue        What is the best way to achieve the same with objects  I d like to do this          a   value 1        b   value 2        c   value 3     reduce function previous  current  index  array     return previous value   current value        However  Object does not seem to have any reduce   method implemented

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1     value 5    value 10   reduce  previousValue  currentValue    gt    return  value  previousValue value   currentValue value      gt  gt  Object  value  15    2     value 5    value 10   map item   gt  item value  reduce  previousValue  currentValue    gt   return previousValue   currentValue      gt  gt  15   3     value 5    value 10   reduce function  previousValue  currentValue          return  value  previousValue value   currentValue value        gt  gt  Object  value  15

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ES6 implementation  Object entries    const o       a   value  1     b   value  2     c   value  3      const total   Object entries o  reduce function  total  pair      const  key  value    pair    return total   value     0

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If handled as an array is much easier Return the total amount of fruits  let fruits      name   banana   id  0  quantity  9      name   strawberry   id  1  quantity  1      name   kiwi   id  2  quantity  2      name   apple   id  3  quantity  4     let total   fruits reduce  sum  f    gt  sum   f quantity  0

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One option would be to reduce the keys     var o          a   value 1        b   value 2        c   value 3       Object keys o  reduce function  previous  key        return previous   o key  value     0     With this  you ll want to specify an initial value or the 1st round will be  a    2   If you want the result as an Object    value          you ll have to initialize and return the object each time   Object keys o  reduce function  previous  key        previous value    o key  value      return previous       value  0

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This is not very difficult to implement yourself   function reduceObj obj  callback  initial         use strict       var key  lastvalue  firstIteration   true      if  typeof callback      function             throw new TypeError callback    is not a function                 if  arguments length  gt  2               initial value set         firstIteration   false          lastvalue   initial            for  key in obj            if   obj hasOwnProperty key   continue          if  firstIteration              firstIteration   false              lastvalue   obj key               continue                    lastvalue   callback lastvalue  obj key   key  obj             if  firstIteration            throw new TypeError  Reduce of empty object with no initial value              return lastvalue      In action   var o    a   value 1   b   value 2   c   value 3    reduceObj o  function prev  curr    prev value    cur value  return prev     value 0    reduceObj o  function prev  curr    return  value  prev value   curr value         both      value  6     reduceObj o  function prev  curr    return prev   curr value     0         6   You can also add it to the Object prototype   if  typeof Object prototype reduce      function         Object prototype reduce   function callback  initial             use strict           var args   Array prototype slice arguments           args unshift this           return reduceObj apply null  args

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First of all  you don t quite get what s reduce s previous value is   In you pseudo code you have return previous value   current value  therefore the previous value will be a number on the next call  not an object   Second  reduce is an Array method  not an Object s one  and you can t rely on the order when you re iterating the properties of an object  see  https   developer mozilla org en-US docs JavaScript Reference Statements for   in  this is applied to Object keys too   so I m not sure if applying reduce over an object makes sense   However  if the order is not important  you can have   Object keys obj  reduce function sum  key        return sum   obj key  value     0     Or you can just map the object s value   Object keys obj  map function key    return this key  value    obj  reduce function  previous  current        return previous   current        P S  in ES6 with the fat arrow function s syntax  already in Firefox Nightly   you could shrink a bit   Object keys obj  map key   gt  obj key  value  reduce  previous  current    gt  previous   current

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Since it hasnt really been confirmed in an answer yet  Underscore s reduce also works for this     reduce        a   value 1        b   value 2        c   value 3      function prev  current         prev is either first object or total value     var total   prev value    prev      return total   current value      Note    reduce will return the only value  object or otherwise  if the list object only has one item  without calling iterator function     reduce        a   value 1      function prev  current         not called       returns  value  1  instead of 1

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An object can be turned into an array with  Object entries    Object keys    Object values    and then be reduced as array  But you can also reduce an object without creating the intermediate array    I ve created a little helper library odict for working with objects    npm install --save odict   It has reduce function that works very much like Array prototype reduce     export const reduce    dict  reducer  accumulator    gt      for  const key in dict      accumulator   reducer accumulator  dict key   key  dict     return accumulator       You could also assign it to   Object reduce   reduce    as this method is very useful   So the answer to your question would be   const result   Object reduce          a   value 1       b   value 2       c   value 3           accumulator  current    gt   accumulator value    current value  accumulator      reducer function must return accumulator    value  0     initial accumulator value

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Extend Object prototype   Object prototype reduce   function  reduceCallback  initialValue         var obj   this  keys   Object keys  obj         return keys reduce  function  prevVal  item  idx  arr             return reduceCallback  prevVal  item  obj item   obj           initialValue         Sample of using   var dataset         key1    value1       key2    value2       key3    value3      function reduceFn  prevVal  key  val  obj         return prevVal   key           val            console log  dataset reduce  reduceFn   initialValue        Output      initialValue  key1   value1  key2   value2  key3   value3       n Joy it  guys     -

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You can use a generator expression  supported in all browsers for years now  and in Node  to get the key-value pairs in a list you can reduce on    gt  gt  gt  a     b   3  Object   b 3    gt  gt  gt    i  a i   for  i in a  if  a hasOwnProperty i       b   3

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Try out this one liner arrow function  Object values o  map a   gt  a value  o  reduce  ac  key  index  arr    gt  ac  key

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If you can use an array  do use an array  the length and order of an array are half its worth   function reducer obj  fun  temp       if typeof fun     function            if temp   undefined  temp              for var p in obj               if obj hasOwnProperty p                    temp  fun obj p   temp  p  obj                                     return temp    var O  a  value 1  b  value 2  c  value 3    reducer O  function a  b  return a value b   0         returned value   Number  6

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What you actually want in this case are the Object values  Here is a concise ES6 implementation with that in mind   const add       a   value 1     b   value 2     c   value 3     const total   Object values add  reduce  t   value     gt  t   value  0   console log total     6   or simply   const add       a  1    b  2    c  3    const total   Object values add  reduce  t  n    gt  t   n   console log total     6

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Try this one  It will sort numbers from other variables   const obj        a  1     b  2     c  3    const result   Object keys obj   reduce  acc  rec    gt  typeof obj rec       number    acc concat  obj rec      acc       reduce  acc  rec    gt  acc   rec

[javascript] Javascript reduce() on Object

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