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

Question

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 rig

Answer 1

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'));

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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'));

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