how to have a nested array with grouping javascript?

I have a base array like below.

const arr1 = [
{id:'1',city:'Sydney',distance:100,yearhalf:'2022_1'},
{id:'2',city:'Melbourne',distance:70,yearhalf:'2022_1'},
{id:'3',city:'Perth',distance:65,yearhalf:'2022_1'},
{id:'4',city:'Sydney',distance:89,yearhalf:'2022_2'},
{id:'5',city:'Melbourne',distance:40,yearhalf:'2022_2'},
{id:'6',city:'Perth',distance:40,yearhalf:'2022_2'}
]

The idea is to group the array elements by "yearhalf".I can achieve that with below approach.

const groupedArray = arr1((acc,item)=>{
        const itemIndex = acc.findIndex(i=>i.yearhalf === item.yearhalf);

        if(itemIndex !== -1){
            acc[itemIndex][item.city]=[item.distance]
        }
        else{
          acc.push({
             [item.city]:[item.distance],
             yearhalf:item.yearhalf
          })
        }
        return acc;
      },[])

The result array will be like below. Currently at this level.

[
{yearhalf:'2022_1',Sydney:[100],Melbourne:[70],Perth:[65]},
{yearhalf:'2022_2',Sydney:[89],Melbourne:[40],Perth:[40]},
]

What I want to do is, if there are multiple occurrences of same "city" and same "yearhalf" combinations(with different id field of course), push only those distances to existing results. So having additional entries like below in the initial array(arr1) would change the above result.

{id:'7',city:'Sydney',distance:50,yearhalf:'2022_1'},
{id:'8',city:'Melbourne',distance:40,yearhalf:'2022_1'}

Since we already have a yearhalf named ‘2022_1’ it should add the city distance to the already existing cityname(Sydney/Melbourne etc) array like below

[
{yearhalf:'2022_1',Sydney:[100,50],Melbourne:[70,40],Perth:[65]},
{yearhalf:'2022_2',Sydney:[89],Melbourne:[40],Perth:[40]},
]

Note: The city name will always be one of Sydney/Melbourne/Perth. Year halves can be have more instances(YYYY_1,YYYY_2) but since initially grouping by them shouldn’t cause much of a problem.

Thanks in advance!

>Solution :

You could check for the array and assign if necessary.

const
    arr1 = [{ id: '1', city: 'Sydney', distance: 100, yearhalf: '2022_1' }, { id: '2',city: 'Melbourne', distance: 70, yearhalf: '2022_1' }, { id: '3', city: 'Perth', distance: 65, yearhalf: '2022_1' }, { id: '4', city: 'Sydney', distance: 89, yearhalf: '2022_2' }, { id: '5', city: 'Melbourne', distance: 40,yearhalf: '2022_2' },  { id: '6', city: 'Perth', distance: 40, yearhalf: '2022_2' }, { id: '7', city: 'Sydney', distance: 50, yearhalf: '2022_1' }, { id: '8', city: 'Melbourne', distance: 40, yearhalf: '2022_1' }],
    groupedArray = arr1.reduce((acc, { city, distance, yearhalf }) => {
        const itemIndex = acc.findIndex(i => i.yearhalf === yearhalf);
        if (itemIndex === -1) {
            acc.push({ yearhalf, [city]: [distance] });
        } else {
            (acc[itemIndex][city] ??= []).push(distance);
        }
        return acc;
    }, []);

console.log(groupedArray);

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An approach without searching.

const
    data = [{ id: '1', city: 'Sydney', distance: 100, yearhalf: '2022_1' }, { id: '2',city: 'Melbourne', distance: 70, yearhalf: '2022_1' }, { id: '3', city: 'Perth', distance: 65, yearhalf: '2022_1' }, { id: '4', city: 'Sydney', distance: 89, yearhalf: '2022_2' }, { id: '5', city: 'Melbourne', distance: 40,yearhalf: '2022_2' },  { id: '6', city: 'Perth', distance: 40, yearhalf: '2022_2' }, { id: '7', city: 'Sydney', distance: 50, yearhalf: '2022_1' }, { id: '8', city: 'Melbourne', distance: 40, yearhalf: '2022_1' }],
    grouped = Object.values(data.reduce((acc, { city, distance, yearhalf }) => {
        acc[yearhalf] ??= { yearhalf };
        (acc[yearhalf][city] ??= []).push(distance);
        return acc;
    }, []));

console.log(grouped);

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