Group by and calculate mean / average of properties in a Javascript array

I struggled finding the solution I was looking for in other stackoverflow posts, even though I strongly feel as if it must exist. If it does, please do forward me in the right direction.

I am trying to do a pretty standard group by in javascript using sports data. I have the following array of objects:

 const myData = [
    {team: "GSW", pts: 120, ast: 18, reb: 11},
    {team: "GSW", pts: 125, ast: 28, reb: 18},
    {team: "GSW", pts: 110, ast: 35, reb: 47},
    {team: "HOU", pts: 100, ast: 17, reb: 43},
    {team: "HOU", pts: 102, ast: 14, reb: 32},
    {team: "SAS", pts: 127, ast: 21, reb: 25},
    {team: "SAS", pts: 135, ast: 25, reb: 37},
    {team: "SAS", pts: 142, ast: 18, reb: 27}
 ]

Each row in my data corresponds to the results of a specific basketball game. Simply put, I would like to group by the data, and apply a mean/average function to the grouped data. The results I would expect are:

const groupedData = [
    {team: "GSW", pts: 118.3, ast: 27.0, reb: 25.3},
    {team: "HOU", pts: 101, ast: 15.5, reb: 37.5},
    {team: "SAS", pts: 134.7, ast: 21.3, reb: 29.7} 
] 

I would prefer to use vanilla javascript with reduce() here... given what I know about reduce, it seems like the best way. I am currently working on this and will post if i can get it to work before someone else posts the answer.

EDIT: My actual data has ~30 keys. I am hoping to find a solution that simply asks me to either (a) specify only the team column to be grouped by, and assume it groups the rest, or (b) pass an array of stat columns (pts, asts, etc.) rather than creating a line for each stat.

Thanks!

Answers:

Answer

One way to do this is to use reduce and map in conjunction.

const myData = [
    {team: "GSW", pts: 120, ast: 18, reb: 11},
    {team: "GSW", pts: 125, ast: 28, reb: 18},
    {team: "GSW", pts: 110, ast: 35, reb: 47},
    {team: "HOU", pts: 100, ast: 17, reb: 43},
    {team: "HOU", pts: 102, ast: 14, reb: 32},
    {team: "SAS", pts: 127, ast: 21, reb: 25},
    {team: "SAS", pts: 135, ast: 25, reb: 37},
    {team: "SAS", pts: 142, ast: 18, reb: 27}
 ]
 
 // Calculate the sums and group data (while tracking count)
 const reduced = myData.reduce(function(m, d){
    if(!m[d.team]){
      m[d.team] = {...d, count: 1};
      return m;
    }
    m[d.team].pts += d.pts;
    m[d.team].ast += d.ast;
    m[d.team].reb += d.reb;
    m[d.team].count += 1;
    return m;
 },{});
 
 // Create new array from grouped data and compute the average
 const result = Object.keys(reduced).map(function(k){
     const item  = reduced[k];
     return {
         team: item.team,
         ast: item.ast/item.count,
         pts: item.pts/item.count,
         reb: item.reb/item.count
     }
 })
 
 console.log(JSON.stringify(result,null,4));

EDIT: Just saw your update to the question. You can do away with each line for each key if you can either whitelist (provide an array of keys to compute) or blacklist (provide an array of keys to ignore) keys to do that programmatically.

const myData = [
    {team: "GSW", pts: 120, ast: 18, reb: 11},
    {team: "GSW", pts: 125, ast: 28, reb: 18},
    {team: "GSW", pts: 110, ast: 35, reb: 47},
    {team: "HOU", pts: 100, ast: 17, reb: 43},
    {team: "HOU", pts: 102, ast: 14, reb: 32},
    {team: "SAS", pts: 127, ast: 21, reb: 25},
    {team: "SAS", pts: 135, ast: 25, reb: 37},
    {team: "SAS", pts: 142, ast: 18, reb: 27}
 ]
 
/**
 * Function which accepts a data array and a list of whitelisted
 * keys to find the average of each key after grouping
 */
function getGroupedData(data, whitelist) {
  // Calculate the sums and group data (while tracking count)
  const reduced = data.reduce(function(m, d) {
    if (!m[d.team]) {
      m[d.team] = { ...d,
        count: 1
      };
      return m;
    }
    whitelist.forEach(function(key) {
      m[d.team][key] += d[key];
    });
    m[d.team].count += 1;
    return m;
  }, {});

  // Create new array from grouped data and compute the average
  return Object.keys(reduced).map(function(k) {
    const item = reduced[k];
    const itemAverage = whitelist.reduce(function(m, key) {
      m[key] = item[key] / item.count;
      return m;
    }, {})
    return {
      ...item, // Preserve any non white-listed keys
      ...itemAverage // Add computed averege for whitelisted keys
    }
  })
}


console.log(JSON.stringify(getGroupedData(myData, ['pts', 'ast', 'reb']), null, 4));

Answer

const myData = [
    {team: "GSW", pts: 120, ast: 18, reb: 11},
    {team: "GSW", pts: 125, ast: 28, reb: 18},
    {team: "GSW", pts: 110, ast: 35, reb: 47},
    {team: "HOU", pts: 100, ast: 17, reb: 43},
    {team: "HOU", pts: 102, ast: 14, reb: 32},
    {team: "SAS", pts: 127, ast: 21, reb: 25},
    {team: "SAS", pts: 135, ast: 25, reb: 37},
    {team: "SAS", pts: 142, ast: 18, reb: 27}
 ]

const groubElement = myData.reduce((obj, val) => {
    if (obj[val.team]) {
        obj[val.team].pts = obj[val.team].pts + val.pts;
        obj[val.team].ast = obj[val.team].pts + val.ast;
        obj[val.team].reb = obj[val.team].pts + val.reb;
        obj[val.team].counter = obj[val.team].counter + 1;
    } else {
        obj[val.team] = val;
        obj[val.team].counter = 1;
    }
    return obj;

}, {});



const groupElementWithMean = Object.values(groubElement).map(({
    counter,
    ...element
}) => {
    element.pts = (element.pts / counter).toFixed(1);
    element.ast = (element.ast / counter).toFixed(1);
    element.reb = (element.reb / counter).toFixed(1);
    return element;
});

console.log(groupElementWithMean);

Answer

You can do this by using reduce with Object.keys and Array.prototype.map as follows:-

const myData = [
    { team: "GSW", pts: 120, ast: 18, reb: 11 },
    { team: "GSW", pts: 125, ast: 28, reb: 18 },
    { team: "GSW", pts: 110, ast: 35, reb: 47 },
    { team: "HOU", pts: 100, ast: 17, reb: 43 },
    { team: "HOU", pts: 102, ast: 14, reb: 32 },
    { team: "SAS", pts: 127, ast: 21, reb: 25 },
    { team: "SAS", pts: 135, ast: 25, reb: 37 },
    { team: "SAS", pts: 142, ast: 18, reb: 27 }
]

let grpData = myData.reduce((acc, cv) => {
    if (!acc[cv.team]) {
        acc[cv.team] = {};
        acc[cv.team].team = cv.team;
        acc[cv.team].count = acc[cv.team].pts = acc[cv.team].ast = acc[cv.team].reb = 0
    }
    acc[cv.team].count++;
    acc[cv.team].pts += cv.pts;
    acc[cv.team].ast += cv.ast;
    acc[cv.team].reb += cv.reb;
    return acc;
}, {});
grpData = Object.keys(grpData).map(key => {
    let { team, reb, ast, pts, count } = grpData[key];
    return {
        team, reb: reb / count, ast: ast / count, pts: pts / count
    };
})
console.log(grpData);

Answer

Using array of statsFields and loop over those to create totals and later to get averages

const myData = [
    {team: "GSW", pts: 120, ast: 18, reb: 11},
    {team: "GSW", pts: 125, ast: 28, reb: 18},
    {team: "GSW", pts: 110, ast: 35, reb: 47},
    {team: "HOU", pts: 100, ast: 17, reb: 43},
    {team: "HOU", pts: 102, ast: 14, reb: 32},
    {team: "SAS", pts: 127, ast: 21, reb: 25},
    {team: "SAS", pts: 135, ast: 25, reb: 37},
    {team: "SAS", pts: 142, ast: 18, reb: 27}
 ]
 
 const statsFields = ['pts','ast','reb'];
 
 const teamsObject = myData.reduce((a,{team,...stats})=>{
   a[team] = a[team] || {team, games:0};
   a[team].games++
   statsFields.forEach(k=> a[team][k] = (a[team][k] || 0) + stats[k]);
   return a;
 },{});
 
 const res = Object.values(teamsObject).map(({games,...team})=>{
    // average for each field total/games
    statsFields.forEach(k=> team[k] = team[k]/games);    
    return team;
 })
 
 console.log(JSON.stringify(res))

Answer

You could take a dynamic approach by using a Map and generate all items after collecting the unknown keys.

function groupBy(array, key) {
    return Array.from(
        array.reduce((m, o) => {
            var temp = m.get(o[key]);
            if (!temp) {
                m.set(o[key], temp = {});
            }
            Object.entries(o).forEach(([k, v]) => {
                if (k === key) {
                    return;
                }
                temp[k] = temp[k]  || { sum: 0, count: 0 };
                temp[k].sum += v;
                temp[k].count++;
            });
            return m;
        }, new Map),
        ([k, v]) => Object.assign({ [key]: k }, ...Object.entries(v).map(([l, { sum, count }]) => ({ [l]: +(sum / count).toFixed(1) })))
    );
}

const myData = [{ team: "GSW", pts: 120, ast: 18, reb: 11 }, { team: "GSW", pts: 125, ast: 28, reb: 18 }, { team: "GSW", pts: 110, ast: 35, reb: 47 }, { team: "HOU", pts: 100, ast: 17, reb: 43 }, { team: "HOU", pts: 102, ast: 14, reb: 32 }, { team: "SAS", pts: 127, ast: 21, reb: 25 }, { team: "SAS", pts: 135, ast: 25, reb: 37 }, { team: "SAS", pts: 142, ast: 18, reb: 27 }];

console.log(groupBy(myData, 'team'));
.as-console-wrapper { max-height: 100% !important; top: 0; }

With rest properties (babel: true).

function groupBy(array, key) {
    return Array.from(
        array.reduce((m, { [key]: k, ...rest}) => {
            var temp = m.get(k);
            if (!temp) {
                m.set(k, temp = {});
            }
            Object.entries(rest).forEach(([l, v]) => {
                temp[l] = temp[l]  || { sum: 0, count: 0 };
                temp[l].sum += v;
                temp[l].count++;
            });
            return m;
        }, new Map),
        ([k, v]) => Object.assign({ [key]: k }, ...Object.entries(v).map(([l, { sum, count }]) => ({ [l]: +(sum / count).toFixed(1) })))
    );
}

const myData = [{ team: "GSW", pts: 120, ast: 18, reb: 11 }, { team: "GSW", pts: 125, ast: 28, reb: 18 }, { team: "GSW", pts: 110, ast: 35, reb: 47 }, { team: "HOU", pts: 100, ast: 17, reb: 43 }, { team: "HOU", pts: 102, ast: 14, reb: 32 }, { team: "SAS", pts: 127, ast: 21, reb: 25 }, { team: "SAS", pts: 135, ast: 25, reb: 37 }, { team: "SAS", pts: 142, ast: 18, reb: 27 }];

console.log(groupBy(myData, 'team'));
.as-console-wrapper { max-height: 100% !important; top: 0; }

Answer

It can be simply done as follows.

Note: Used JSON.parse and stringify to deep shallow copy the data. Or else the original array gets modified. Its not needed if the original array can be modified.

const data = [
    {team: "GSW", pts: 120, ast: 18, reb: 11},
    {team: "GSW", pts: 125, ast: 28, reb: 18},
    {team: "GSW", pts: 110, ast: 35, reb: 47},
    {team: "HOU", pts: 100, ast: 17, reb: 43},
    {team: "HOU", pts: 102, ast: 14, reb: 32},
    {team: "SAS", pts: 127, ast: 21, reb: 25},
    {team: "SAS", pts: 135, ast: 25, reb: 37},
    {team: "SAS", pts: 142, ast: 18, reb: 27}
 ];

function groupData(mydata,keys)
{
    var accresult = mydata.reduce(function(acc, value){
      var arr = acc.filter(function(obj){return obj.team==value.team});
      arr.length ? (item=arr[0] , keys.forEach(function(key){ item[key]+=value[key]; })) : acc.push(value);
      return acc;
  },[]);

  var result = accresult.map(function(val){
      var l = mydata.filter(function(obj){return obj.team==val.team}).length;
      keys.forEach(function(key){ val[key]=(val[key]/l).toFixed(2); })
      return val;
  });
  return result;
}

console.log(groupData(JSON.parse(JSON.stringify(data.slice(0))),['pts','ast']));
console.log(groupData(JSON.parse(JSON.stringify(data.slice(0))),['pts','ast','reb']));
console.log(groupData(JSON.parse(JSON.stringify(data.slice(0))),['pts']));

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