Javascript reduce on array of objects

Question

Say I want to sum a x for each element in arr   arr     x 1   x 2   x 4   arr reduce function a b  return a x   b x    gt  gt  NaN   I have cause to believe that a x is undefined at some point   The following works fine  arr    1 2 4  arr reduce function a b  return a   b    gt  gt  7   What am I doing wrong in the first example

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In the first step  it will work fine as the value of a will be 1 and that of b will be 2 but as 2 1 will be returned and in the next step the value of b will be the return value from step 1 i e 3 and so b x will be undefined   and undefined   anyNumber will be NaN and that is why you are getting that result   Instead you can try this by giving initial value as zero i e  arr reduce function a b  return a   b x  0

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I used to encounter this is my development  what I do is wrap my solution in a function to make it reusable in my environment  like this  const sumArrayOfObject   array  prop   gt array reduce  sum  n   gt  return sum   n prop    0

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You could use the map and reduce functions  const arr     x 1   x 2   x 4    const sum   arr map n   gt  n x  reduce  a  b    gt  a   b  0

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At each step of your reduce  you aren t returning a new  x      object  So you either need to do   arr     x 1   x 2   x 4   arr reduce function a b  return a   b x     or you need to do  arr     x 1   x 2   x 4   arr reduce function a b  return  x  a x   b x

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reduce function iterates over a collection  arr     x 1   x 2   x 4      is a collection  arr reduce function a b  return a x   b x     translates to   arr reduce        for each index in the collection  this callback function is called   function       a    a   accumulator  during each callback   value of accumulator is           passed inside the variable  a      b    currentValue   for ex currentValue is  x 1  in 1st callback     currentIndex      array           return a x   b x          accumulator    this is returned at the end of arr reduce call        accumulator   returned value i e return a x   b x  in each callback           during each index callback  value of variable  accumulator  is   passed into  a  parameter in the callback function  If we don t initialize  accumulator   its value will be undefined  Calling undefined x would give you error        To solve this  initialize  accumulator  with value 0 as Casey s answer showed above    To understand the in-outs of  reduce  function  I would suggest you look at the source code of this function  Lodash library has reduce function which works exactly same as  reduce  function in ES6   Here is the link   reduce source code

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If you have a complex object with a lot of data  like an array of objects  you can take a step by step approach to solve this   For e g   const myArray      id  1  value  10     id  2  value  20      First  you should map your array into a new array of your interest  it could be a new array of values in this example   const values   myArray map obj   gt  obj value     This call back function will return a new array containing only values from the original array and store it on values const  Now your values const is an array like this   values    10  20     And now your are ready to perform your reduce   const sum   values reduce  accumulator  currentValue    gt    return accumulator   currentValue      0     As you can see  the reduce method executes the call back function multiple times  For each time  it takes the current value of the item in the array and sum with the accumulator  So to properly sum it you need to set the initial value of your accumulator as the second argument of the reduce method   Now you have your new const sum with the value of 30

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To formalize what has been pointed out  a reducer is a catamorphism which takes two arguments which may be the same type by coincidence  and returns a type which matches the first argument   function reducer  accumulator  X  currentValue  Y   X       That means that the body of the reducer needs to be about converting currentValue and the current value of the accumulator to the value of the new accumulator   This works in a straightforward way  when adding  because the accumulator and the element values both happen to be the same type  but serve different purposes     1  2  3  reduce  x  y    gt  x   y     This just works because they re all numbers      age  5      age  2      age  8       reduce  total  thing    gt  total   thing age  0     Now we re giving a starting value to the aggregator  The starting value should be the type that you expect the aggregator to be  the type you expect to come out as the final value   in the vast majority of cases  While you aren t forced to do this  and shouldn t be   it s important to keep in mind   Once you know that  you can write meaningful reductions for other n 1 relationship problems   Removing repeated words   const skipIfAlreadyFound    words  word    gt  words includes word        words       words concat word    const deduplicatedWords   aBunchOfWords reduce skipIfAlreadyFound         Providing a count of all words found   const incrementWordCount    counts  word    gt      counts word     counts word     0    1    return counts     const wordCounts   words reduce incrementWordCount          Reducing an array of arrays  to a single flat array   const concat    a  b    gt  a concat b    const numbers        1  2  3      4  5  6      7  8  9    reduce concat         Any time you re looking to go from an array of things  to a single value that doesn t match a 1 1  reduce is something you might consider   In fact  map and filter can both be implemented as reductions   const map    transform  array    gt    array reduce  list  el    gt  list concat transform el          const filter    predicate  array    gt  array reduce     list  el    gt  predicate el    list concat el    list            I hope this provides some further context for how to use reduce   The one addition to this  which I haven t broken into yet  is when there is an expectation that the input and output types are specifically meant to be dynamic  because the array elements are functions   const compose       fns    gt  x   gt    fns reduceRight  x  f    gt  f x   x    const hgfx   h g f x     const hgf   compose h  g  f   const hgfy   hgf y   const hgfz   hgf z

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I did it in ES6 with a little improvement  arr reduce  a  b    gt    x  a x   b x    x  return number

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A cleaner way to accomplish this is by providing an initial value    x000D   x000D  var arr     x 1    x 2    x 4    x000D  arr reduce function  acc  obj    return acc   obj x     0      7 x000D  console log arr   x000D   x000D   x000D    The first time the anonymous function is called  it gets called with  0   x  1   and returns 0   1   1  The next time  it gets called with  1   x  2   and returns 1   2   3  It s then called with  3   x  4    finally returning 7

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to return a sum of all x props   arr reduce   a b    gt   a x    a    b x

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You can use reduce method as bellow  If you change the 0 zero  to 1 or other numbers  it will add it to total number  For example  this example gives the total number as 31 however if we change 0 to 1  total number will be 32  const batteryBatches    4  5  3  4  4  6  5    let totalBatteries  batteryBatches reduce  acc val    gt  acc   val  0

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For the first iteration  a  will be the first object in the array  hence a x   b x will return 1 2 i e 3  Now in the next iteration the returned 3 is assigned to a  so a is a number now n calling a x will give NaN   Simple solution is first mapping the numbers in array and then reducing them as below    arr map a  gt a x  reduce function a b  return a b     here arr map a  gt a x  will provide an array of numbers  1 2 4  now using  reduce function a b  return a b   will simple add these numbers without any hassel  Another simple solution is just to provide an initial sum as zero by assigning 0 to  a  as below   arr reduce function a b  return a   b x  0

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After the first iteration your re returning a number and then trying to get property x of it to add to the next object which is undefined and maths involving undefined results in NaN    try returning an object contain an x property with the sum of the x properties of the parameters   var arr     x 1   x 2   x 4     arr reduce function  a  b      return  x  a x   b x      returns object with property x        ES6 arr reduce  a  b    gt    x  a x   b x         - gt   x  7    Explanation added from comments   The return value of each iteration of    reduce used as the a variable in the next iteration    Iteration 1  a    x 1   b    x 2    x  3  assigned to a in Iteration 2  Iteration 2  a    x 3   b    x 4     The problem with your example is that you re returning a number literal   function  a  b      return a x   b x     returns number literal     Iteration 1  a    x 1   b    x 2      returns 3 as a in next iteration  Iteration 2  a   3  b    x 2  returns NaN  A number literal 3 does not  typically  have a property called x so it s undefined and  undefined   b x returns NaN and NaN    lt anything gt  is always NaN  Clarification  I prefer my method over the other top answer in this thread as I disagree with the idea that passing an optional parameter to reduce with a magic number to get out a number primitive is cleaner  It may result in fewer lines written but imo it is less readable

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Array reduce function takes three parameters i e  initialValue default  it s 0    accumulator and current value   By default the value of initialValue will be  0    which is taken by  accumulator     Let s see this in code    var arr   1 2 4    arr reduce  acc currVal    gt  acc   currVal          remember Initialvalue is 0 by default      first iteration     0  1   gt  Now accumulator  1    second iteration     1  2   gt  Now accumulator  3    third iteration     3   4   gt  Now accumulator   7  No more array properties now the loop breaks      solution   7   Now same example with initial Value    var initialValue   10  var arr   1 2 4    arr reduce  acc currVal    gt  acc   currVal initialValue            remember Initialvalue is 0 by default but now it s 10      first iteration     10  1   gt  Now accumulator  11    second iteration     11  2   gt  Now accumulator  13    third iteration     13   4   gt  Now accumulator   17  No more array properties now the loop breaks     solution 17   Same applies for the object arrays as well the current stackoverflow question     var arr     x 1   x 2   x 4   arr reduce function acc currVal  return acc   currVal x       destructing  x 1    currVal  Now currVal is object which have all the object properties  So now  currVal x  gt 1    first iteration     0  1   gt  Now accumulator  1    second iteration     1  2   gt  Now accumulator  3    third iteration     3   4   gt  Now accumulator   7  No more array properties now the loop breaks    solution 7   ONE THING TO BARE IN MIND is InitialValue by default is 0 and can be given anything i mean       and number

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TL DR  set the initial value Using destructuring arr reduce    sum    x       gt  sum   x   0  Without destructuring arr reduce    sum   cur     gt  sum   cur x   0  With Typescript arr reduce    sum    x       x  number       gt  sum   x   0  Let s try the destructuring method   x000D   x000D  const arr       x  1      x  2      x  4     const result   arr reduce    sum    x       gt  sum   x   0  console log  result      7 x000D   x000D   x000D   The key to this is setting initial value  The return value becomes first parameter of the next iteration  Technique used in top answer is not idiomatic The accepted answer proposes NOT passing the  quot optional quot  value  This is wrong  as the idiomatic way is that the second parameter always be included  Why  Three reasons  1  Dangerous -- Not passing in the initial value is dangerous and can create side-effects and mutations if the callback function is careless  Behold const badCallback    a i    gt  Object assign a i    const foo       a  1      b  2      c  3     const bar   foo reduce  badCallback       bad use of Object assign    Look  we ve tampered with the original array foo         a  1  b  2  c  3      b  2      c  3      If however we had done it this way  with the initial value  const bar   foo reduce  badCallback         foo is still OK foo      a  1  b  2  c  3    For the record  unless you intend to mutate the original object  set the first parameter of Object assign to an empty object  Like this  Object assign     a  b  c   2 - Better Type Inference --When using a tool like Typescript or an editor like VS Code  you get the benefit of telling the compiler the initial and it can catch errors if you re doing it wrong  If you don t set the initial value  in many situations it might not be able to guess and you could end up with creepy runtime errors  3 - Respect the Functors -- JavaScript shines best when its inner functional child is unleashed  In the functional world  there is a standard on how you  quot fold quot  or reduce an array  When you fold or apply a catamorphism to the array  you take the values of that array to construct a new type  You need to communicate the resulting type--you should do this even if the final type is that of the values in the array  another array  or any other type  Let s think about it another way  In JavaScript  functions can be pass around like data  this is how callbacks work  what is the result of the following code   1 2 3  reduce callback  Will it return an number  An object  This makes it clearer  1 2 3  reduce callback 0  Read more on the functional programming spec here  https   github com fantasyland fantasy-land foldable Some more background The reduce method takes two parameters  Array prototype reduce  callback  initialItem    The callback function takes the following parameters  accumulator  itemInArray  indexInArray  entireArray    gt       do stuff       For the first iteration   If initialItem is provided  the reduce function passes the initialItem as the accumulator and the first item of the array as the itemInArray   If initialItem is not provided  the reduce function passes the first item in the array as the initialItem and the second item in the array as itemInArray which can be confusing behavior    I teach and recommend always setting the initial value of reduce  You can check out the documentation at  https   developer mozilla org en-US docs Web JavaScript Reference Global Objects Array Reduce Hope this helps

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x000D   x000D       x000D     x000D     fill creates array with n element x000D     reduce requires 2 parameter   3rd parameter as a length x000D   var fibonacci    n    gt  Array n  fill   reduce  a  b  c    gt    x000D        return a concat c  lt  2   c   a c - 1    a c - 2   x000D           x000D    console log fibonacci 8   x000D   x000D   x000D

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Others have answered this question  but I thought I d toss in another approach  Rather than go directly to summing a x  you can combine a map  from a x to x  and reduce  to add the x s    arr     x 1   x 2   x 4   arr map function a   return a x        reduce function a b   return a   b       Admittedly  it s probably going to be slightly slower  but I thought it worth mentioning it as an option

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you should not use a x for accumulator   Instead you can do like this                       arr     x 1   x 2   x 4    arr reduce function a b  a   b x  0

[javascript] Javascript reduce on array of objects

Instead you can try this by giving initial value as zero i.e

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