How to group an array of objects by key

Question

Does anyone know of a  lodash if possible too  way to group an array of objects by an object key then create a new array of objects based on the grouping  For example  I have an array of car objects   var cars                    make    audi            model    r8            year    2012                    make    audi            model    rs5            year    2013                    make    ford            model    mustang            year    2012                    make    ford            model    fusion            year    2015                    make    kia            model    optima            year    2012              I want to make a new array of car objects that s grouped by make   var cars          audi                            model    r8                year    2012                            model    rs5                year    2013                          ford                            model    mustang                year    2012                            model    fusion                year    2015                         kia                            model    optima                year    2012

User · Answer

There is absolutely no reason to download a 3rd party library to achieve this simple problem, like the above solutions suggest.

The one line version to group a list of objects by a certain key in es6:

const groupByKey = (list, key) => list.reduce((hash, obj) => ({...hash, [obj[key]]:( hash[obj[key]] || [] ).concat(obj)}), {})

The longer version that filters out the objects without the key:

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function groupByKey(array, key) {
   return array
     .reduce((hash, obj) => {
       if(obj[key] === undefined) return hash; 
       return Object.assign(hash, { [obj[key]]:( hash[obj[key]] || [] ).concat(obj)})
     }, {})
}


var cars = [{'make':'audi','model':'r8','year':'2012'},{'make':'audi','model':'rs5','year':'2013'},{'make':'ford','model':'mustang','year':'2012'},{'make':'ford','model':'fusion','year':'2015'},{'make':'kia','model':'optima','year':'2012'}];

console.log(groupByKey(cars, 'make'))

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NOTE: It appear the original question asks how to group cars by make, but omit the make in each group. So the short answer, without 3rd party libraries, would look like this:

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const groupByKey = (list, key, {omitKey=false}) => list.reduce((hash, {[key]:value, ...rest}) => ({...hash, [value]:( hash[value] || [] ).concat(omitKey ? {...rest} : {[key]:value, ...rest})} ), {})

var cars = [{'make':'audi','model':'r8','year':'2012'},{'make':'audi','model':'rs5','year':'2013'},{'make':'ford','model':'mustang','year':'2012'},{'make':'ford','model':'fusion','year':'2015'},{'make':'kia','model':'optima','year':'2012'}];

console.log(groupByKey(cars, 'make', {omitKey:true}))

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User · Answer

Timo s answer is how I would do it  Simple   groupBy  and allow some duplications in the objects in the grouped structure    However the OP also asked for the duplicate make keys to be removed  If you wanted to go all the way     var grouped     mapValues   groupBy cars   make                              clist   gt  clist map car   gt    omit car   make       console log grouped     Yields     audi         model   r8   year   2012            model   rs5   year   2013         ford         model   mustang   year   2012            model   fusion   year   2015         kia      model   optima   year   2012          If you wanted to do this using Underscore js  note that its version of   mapValues is called   mapObject

User · Answer

For cases where key can be null and we want to group them as others   var cars      make   audi   model   r8   year   2012     make   audi   model   rs5   year   2013     make   ford   model   mustang   year   2012     make   ford   model   fusion   year   2015     make   kia   model   optima   year   2012                  make   kia   model   optima   year   2033                  make  null  model   zen   year   2012                  make  null  model   blue   year   2017                      result   cars reduce function  r  a            key   a make     others           r key    r key                 r key  push a           return r         Object create null

User · Answer

I made a benchmark to test the performance of each solution that don t use external libraries  JSBen ch The reduce   option  posted by  Nina Scholz seems to be the optimal one

User · Answer

Prototype version using ES6 as well  Basically this uses the reduce function to pass in an accumulator and current item  which then uses this to build your  grouped  arrays based on the passed in key  the inner part of the reduce may look complicated but essentially it is testing to see if the key of the passed in object exists and if it doesn t then create an empty array and append the current item to that newly created array otherwise using the spread operator pass in all the objects of the current key array and append current item  Hope this helps someone    Array prototype groupBy   function k      return this reduce  acc  item    gt    acc item k          acc item k           item    acc           const projs             project   A       timeTake  2      desc   this is a description               project   B       timeTake  4      desc   this is a description               project   A       timeTake  12      desc   this is a description               project   B       timeTake  45      desc   this is a description          console log projs groupBy  project

User · Answer

x000D   x000D  var cars        make   audi     model   r8     year   2012         make   audi     model   rs5     year   2013         make   ford     model   mustang     year   2012         make   ford     model   fusion     year   2015         make   kia     model   optima     year   2012     reduce  r  car    gt       const       model      year      make       car     r make        r make               model      year          return r           console log cars   x000D   x000D   x000D

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const reGroup    list  key    gt        const newGroup           list forEach item   gt            const newItem   Object assign     item           delete newItem key           newGroup item key     newGroup item key                  newGroup item key   push newItem               return newGroup       const animals             type   dog       breed   puddle               type   dog       breed   labradoodle               type   cat       breed   siamese               type   dog       breed   french bulldog               type   cat       breed   mud         console log reGroup animals   type       const cars                make    audi          model    r8          year    2012                make    audi          model    rs5          year    2013                make    ford          model    mustang          year    2012                make    ford          model    fusion          year    2015                make    kia          model    optima          year    2012           console log reGroup cars   make

User · Answer

You are looking for   groupBy     Removing the property you are grouping by from the objects should be trivial if required    x000D   x000D  var cars      make   audi   model   r8   year   2012     make   audi   model   rs5   year   2013     make   ford   model   mustang   year   2012     make   ford   model   fusion   year   2015     make   kia   model   optima   year   2012      x000D   x000D  var grouped     groupBy cars  function car    x000D    return car make  x000D      x000D   x000D  console log grouped   x000D   lt script src  https   cdn jsdelivr net lodash 4 17 2 lodash min js  gt  lt  script gt  x000D   x000D   x000D      As a bonus  you get even nicer syntax with ES6 arrow functions   const grouped     groupBy cars  car   gt  car make

User · Answer

Building on the answer by  Jonas Wilms if you do not want to type in all your fields       var result            for   let   first field     fields   of your data                  result first field    result first field                result first field  push      fields               I didn t make any benchmark but I believe using a for loop would be more efficient than anything suggested in this answer as well

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I d leave REAL GROUP BY for JS Arrays example exactly the same this task here   x000D   x000D  const inputArray      x000D        Phase   Phase 1   Step   Step 1   Task   Task 1   Value   5     x000D        Phase   Phase 1   Step   Step 1   Task   Task 2   Value   10     x000D        Phase   Phase 1   Step   Step 2   Task   Task 1   Value   15     x000D        Phase   Phase 1   Step   Step 2   Task   Task 2   Value   20     x000D        Phase   Phase 2   Step   Step 1   Task   Task 1   Value   25     x000D        Phase   Phase 2   Step   Step 1   Task   Task 2   Value   30     x000D        Phase   Phase 2   Step   Step 2   Task   Task 1   Value   35     x000D        Phase   Phase 2   Step   Step 2   Task   Task 2   Value   40    x000D     x000D   x000D  var outObject   inputArray reduce function a  e    x000D       GROUP BY estimated key  estKey   well  may be a just plain key x000D       a -- Accumulator result object x000D       e -- sequentally checked Element  the Element that is tested just at this itaration x000D   x000D       new grouping name may be calculated  but must be based on real value of real field x000D    let estKey    e  Phase      x000D   x000D     a estKey    a estKey     a estKey    null         push e   x000D    return a  x000D          x000D   x000D  console log outObject   x000D   x000D   x000D

User · Answer

I liked  metakunfu answer  but it doesn t provide the expected output exactly  Here s an updated that get rid of  make  in the final JSON payload   var cars                    make    audi            model    r8            year    2012                    make    audi            model    rs5            year    2013                    make    ford            model    mustang            year    2012                    make    ford            model    fusion            year    2015                    make    kia            model    optima            year    2012             result   cars reduce  h  car    gt  Object assign h     car make    h car make          concat  model  car model  year  car year            console log JSON stringify result      Output           audi                          model   r8             year   2012                               model   rs5             year   2013                    ford                          model   mustang             year   2012                               model   fusion             year   2015                    kia                          model   optima             year   2012

User · Answer

Here is a solution inspired from Collectors groupingBy   in Java    x000D   x000D  function groupingBy list  keyMapper    x000D    return list reduce  accummalatorMap  currentValue    gt    x000D      const key   keyMapper currentValue   x000D      if  accummalatorMap has key     x000D        accummalatorMap set key   currentValue    x000D        else   x000D        accummalatorMap set key  accummalatorMap get key  push currentValue    x000D        x000D      return accummalatorMap  x000D       new Map     x000D    x000D   x000D   x000D    This will give a Map object    x000D   x000D     Usage x000D   x000D  const carMakers   groupingBy cars  car   gt  car make   x000D   x000D   x000D

User · Answer

You can also make use of array forEach   method like this    x000D   x000D  const cars      make   audi   model   r8   year   2012       make   audi   model   rs5   year   2013       make   ford   model   mustang   year   2012       make   ford   model   fusion   year   2015       make   kia   model   optima   year   2012      x000D   x000D  let newcars      x000D   x000D  cars forEach car   gt    x000D    newcars car make       check if that array exists or not in newcars object x000D      newcars car make  push  model  car model  year  car year       just push x000D        newcars car make        newcars car make  push  model  car model  year  car year       create a new array and push x000D     x000D   x000D  console log newcars   x000D   x000D   x000D

User · Answer

In plain Javascript  you could use Array reduce with an object   x000D   x000D  var cars      make   audi   model   r8   year   2012       make   audi   model   rs5   year   2013       make   ford   model   mustang   year   2012       make   ford   model   fusion   year   2015       make   kia   model   optima   year   2012      x000D      result   cars reduce function  r  a    x000D          r a make    r a make         x000D          r a make  push a   x000D          return r  x000D         Object create null    x000D   x000D  console log result   x000D   as-console-wrapper   max-height  100   important  top  0    x000D   x000D   x000D

User · Answer

Here is your very own groupBy function which is a generalization of the code from  https   github com you-dont-need You-Dont-Need-Lodash-Underscore   x000D   x000D  function groupBy xs  f    x000D    return xs reduce  r  v  i  a  k   f v     gt    r k      r k         push v   r        x000D    x000D   x000D  const cars      make   audi   model   r8   year   2012       make   audi   model   rs5   year   2013       make   ford   model   mustang   year   2012       make   ford   model   fusion   year   2015       make   kia   model   optima   year   2012      x000D   x000D  const result   groupBy cars   c    gt  c make   x000D  console log result   x000D   x000D   x000D

User · Answer

Grouped Array of Object in typescript with this   groupBy  list  any    key  string   Map lt string  Array lt any gt  gt        let map   new Map        list map val  gt            if  map has val key                 map set val key  list filter data   gt  data key     val key                           return map

User · Answer

Agree that unless you use these  often there is no need for an external library  Although similar solutions are available  I see that some of them are tricky to follow here is a gist that has a solution with comments if you re trying to understand what is happening   x000D   x000D  const cars         make    audi      model    r8      year    2012          make    audi      model    rs5      year    2013          make    ford      model    mustang      year    2012          make    ford      model    fusion      year    2015          make    kia      model    optima      year    2012                Groups an array of objects by a key an returns an object or array grouped by provided key      param array - array to group objects by key      param key - key to group array objects by      param removeKey  - remove the key and it s value from the resulting object      param outputType - type of structure the output should be contained in      const groupBy       inputArray    key    removeKey   false    outputType           gt      return inputArray reduce       previous  current    gt             Get the current value that matches the input key and remove the key value for it        const            key   keyValue           current           remove the key if option is set       removeKey  amp  amp  keyValue  amp  amp  delete current key            If there is already an array for the user provided key use it else default to an empty array        const            keyValue   reducedValue                previous            Create a new object and return that merges the previous with the current object       return Object assign previous             keyValue   reducedValue concat current                          Replace the object here to an array to change output object to an array     outputType           console log groupBy cars   make   true   x000D   x000D   x000D

User · Answer

Create a method which can be re-used  Array prototype groupBy   function prop          return this reduce function groups  item            const val   item prop          groups val    groups val                groups val  push item          return groups                       Then below you can group by any criteria  const groupByMake   cars groupBy  make            console log groupByMake      x000D   x000D  var cars     x000D        x000D           make    audi   x000D           model    r8   x000D           year    2012  x000D           x000D           make    audi   x000D           model    rs5   x000D           year    2013  x000D           x000D           make    ford   x000D           model    mustang   x000D           year    2012  x000D           x000D           make    ford   x000D           model    fusion   x000D           year    2015  x000D           x000D           make    kia   x000D           model    optima   x000D           year    2012  x000D         x000D     x000D      re-usable method x000D  Array prototype groupBy   function prop    x000D     return this reduce function groups  item    x000D    const val   item prop  x000D    groups val    groups val        x000D    groups val  push item  x000D    return groups x000D            x000D      x000D     x000D      initiate your groupBy  Notice the recordset Cars and the field Make     x000D    const groupByMake   cars groupBy  make    x000D    console log groupByMake   x000D       x000D        At this point we have objects  You can use Object keys to return an array x000D   x000D   x000D

User · Answer

You can try to modify the object inside the function called per iteration by   groupBy func   Notice that the source array change his elements   var res     groupBy cars  car   gt       const makeValue car make      delete car make      return makeValue     console log res   console log cars

User · Answer

I love to write it with no dependency complexity just pure simple js   x000D   x000D  const mp      const cars             model   Imaginary space craft SpaceX model       year   2025               make   audi       model   r8       year   2012               make   audi       model   rs5       year   2013               make   ford       model   mustang       year   2012               make   ford       model   fusion       year   2015               make   kia       model   optima       year   2012         cars forEach c   gt      if   c make  return    exit  maybe add them to a  no make  category     if   mp c make   mp c make       model  c model  year  c year      else mp c make  push   model  c model  year  c year        console log mp  x000D   x000D   x000D

User · Answer

With lodash fp you can create a function with   flow   that 1st groups by a key  and then map each group  and omits a key from each item    x000D   x000D  const   flow  groupBy  mapValues  map  omit        x000D   x000D  const groupAndOmitBy   key   gt  flow  x000D    groupBy key   x000D    mapValues map omit key    x000D     x000D   x000D  const cars      make   audi   model   r8   year   2012       make   audi   model   rs5   year   2013       make   ford   model   mustang   year   2012       make   ford   model   fusion   year   2015       make   kia   model   optima   year   2012      x000D   x000D  const groupAndOmitMake   groupAndOmitBy  make    x000D   x000D  const result   groupAndOmitMake cars   x000D   x000D  console log result   x000D   as-console-wrapper   max-height  100   important  top  0    x000D   lt script src  https   cdn jsdelivr net g lodash 4 lodash min js lodash fp min js   gt  lt  script gt  x000D   x000D   x000D

User · Answer

function groupBy data  property      return data reduce  acc  obj    gt        const key   obj property       if   acc key           acc key                  acc key  push obj       return acc              groupBy people   age

User · Answer

Its also possible with a simple for loop    const result         for const  make  model  year  of cars       if  result make   result make           result make  push   model  year

User · Answer

Just try this one it works fine for me  let grouped     groupBy cars   make    Note  Using lodash lib  so include it

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Here is another solution to it  As requested      I want to make a new array of car objects that s grouped by make    function groupBy       const key    make     return cars reduce  acc  x    gt            acc       x key      acc x key               model  x model        year  x year              acc x key            model  x model        year  x year                      Output   console log  Grouped by make key   groupBy

[javascript] How to group an array of objects by key

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