How to group by, calculate sum and get average in JavaScript array?

Question

I have array of object

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

Expected result:

//sum
 sumAge = {
 editor: 57,  // 23+34
 admin: 56    // 28+28
}

//average
avgAge = {
   editor: 28.5,  // (23+34) / 2
   admin: 28    //(28+28)/2
}

I use reduce() method to group the objects in an array by 'group' and calulate 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: 57, admin: 56} 
done!

How to group object of Array by key 'group' and calulate 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: 28.5}
    {group: "admin", ageAvg: 28}
]

But Expected result:

result = {
   editor: 28.5,  // (23+34) / 2
   admin: 28    //(28+28)/2
}

Does this answer your question? calculate sum, average array of object in js use reduce or for loop — ggorlen
– ggorlen, Commented Aug 12, 2020 at 18:20

Always Helping · Accepted Answer · 2020-08-12 11:12:24Z

2

You could simply use reduce to get the total of age groups.

And use object.keys length to get the average of your total as new object from getAvg function.

Demo:

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

const sumId = users.reduce((a, {
  group,
  age
}) => (a[group] = (a[group] || 0) + age, a), {});

console.log(sumId); //{editor: 57, admin: 56}

//Average
const getAvg = (x) => {
  const item = {}
  const count = Object.keys(x).length
  Object.keys(x).map(function(y) {
    item[y] = sumId[y] / count
  })
  return item
}
console.log(getAvg(sumId)); //{editor: 28.5, admin: 28}

edited Aug 12, 2020 at 11:12

answered Aug 12, 2020 at 10:48

Always Helping

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2 Comments

minhminh Over a year ago

sorry guy, you calulate average wrong. example: jsfiddle.net/pw9cuer4/1

Always Helping Over a year ago

@minhminh I NO calculate average wrong. I exactly give solution as you said in the expected output in your question. Working: jsfiddle.net/pmefb80L

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