calculate sum, average array of object in js use reduce or for loop [duplicate]

Question

I have array of object

 const users = [
     { group: 'editor', name: 'Adam', age: 23 },
     { group: 'editor', name: 'John', age: 28 },
     { group: 'editor', name: 'William', age: 34 },
     { group: 'admin', name: 'Oliver', age: 28' }
 ];

Expected result:

//sum
 sumAge = {
 editor: 85,  // 23+34+28
 admin: 28    // 28
}

//average
avgAge = {
   editor: 28.33,  // (85) / 3
   admin: 28    //(28)/1
}

I use reduce() method to group the objects in an array by 'group' and calculate sum:

let sumAge = users.reduce((group, age) => {
    group[age.group] = (group[age.group] || 0) + age.age || 1;
    return group;
}, {})
console.log('sumAge', sumAge); // sumAge: {editor: 85, admin: 28} 
done!

How to group object of Array by key 'group' and calculate average?. I tried:

let ageAvg= users.reduce((group, age) => {
      if (!group[age.group]) {
      group[age.group] = { ...age, count: 1 }
         return group;
      }
      group[age.group].age+= age.age;
      group[age.group].count += 1;
      return group;
      }, {})
const result = Object.keys(ageAvg).map(function(x){
     const item  = ageAvg[x];
     return {
         group: item.group,
         ageAvg: item.age/item.count,
     }
 })
console.log('result',result);
/*
result=[
    {group: "editor", ageAvg: 28.33}
    {group: "admin", ageAvg: 28}
]

But Expected result:

result = {
   editor: 28.33,  // (85) / 3
   admin: 28    //(28)/1
}

If you want a single object, map() will not work for you. And if you want just group: average, then your return inside that map doesn't match that at all — Taplar
– Taplar, Commented Aug 12, 2020 at 17:05
Otherwise, reduce() the result of that map to the single object you want — Taplar
– Taplar, Commented Aug 12, 2020 at 17:05

ggorlen · Accepted Answer · 2020-08-12 17:07:58Z

3

You can reduce to combine age values per group, then map to compute averages per key and convert back to an object with Object.fromEntries:

const users = [
  { group: 'editor', name: 'Adam', age: 23 },
  { group: 'editor', name: 'John', age: 28 },
  { group: 'editor', name: 'William', age: 34 },
  { group: 'admin', name: 'Oliver', age: 28 }
];

const grouped = users.reduce((a, e) => {
  if (!a[e.group]) {
    a[e.group] = [];
  }
  
  a[e.group].push(e.age);
  return a;
}, {});

const avgs = Object.fromEntries(
  Object.entries(grouped).map(([k, v]) => [
    k, v.reduce((a, e) => a + e, 0) / v.length
  ])
);

console.log(avgs);

answered Aug 12, 2020 at 17:07

ggorlen

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5 Comments

minhminh Over a year ago

Thanks for replying to my post... I don't understand code : const avgs = Object.fromEntries( Object.entries(grouped).map(([k, v]) => [ k, v.reduce((a, e) => a + e, 0) / v.length ]) );

ggorlen Over a year ago

What about it don't you understand? Object.entries dumps an object into an array of pairs, map's callback takes the average of each array and Object.fromEntries converts the entry pairs back into an object. Please see the docs for each function, and/or split it into multiple intermediate variables and print each one as you go.

Stephen P Over a year ago

@ggorlen - you point out this is a duplicate of minhminh's previous post; how about putting this answer there and help to close this dupe?

ggorlen Over a year ago

@StephenP I didn't realize it was a dupe until after I'd posted my answer. I'll vote to delete whichever post gets closed first.

Always Helping Over a year ago

@ggorlen. OP is posting this question multiple times - in this question i answered exactly that was mentioned in the expected output here.

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