Group an array of objects by a key

JavaScript version

const groupBy = (arr, key) => arr.reduce((acc, item) => ((acc[item[key]] = [...(acc[item[key]] || []), item]), acc), {});

TypeScript version

const groupBy = <T extends Record<string, any>, K extends keyof T>(arr: T[], key: K): Record<string, T[]> => (
    arr.reduce((acc, item) => ((acc[item[key]] = [...(acc[item[key]] || []), item]), acc), {} as Record<string, T[]>)
);

Example

groupBy(
    [
        { branch: 'audi', model: 'q8', year: '2019' },
        { branch: 'audi', model: 'rs7', year: '2020' },
        { branch: 'ford', model: 'mustang', year: '2019' },
        { branch: 'ford', model: 'explorer', year: '2020' },
        { branch: 'bmw', model: 'x7', year: '2020' },
    ],
    'branch'
);

/*
{
    audi: [
        { branch: 'audi', model: 'q8', year: '2019' },
        { branch: 'audi', model: 'rs7', year: '2020' }
    ],
    bmw: [
        { branch: 'bmw', model: 'x7', year: '2020' }
    ],
    ford: [
        { branch: 'ford', model: 'mustang', year: '2019' },
        { branch: 'ford', model: 'explorer', year: '2020' }
    ],
}
*/