GitHub 6219★

Count by the properties of an array of objects

JavaScript version

const countBy = (arr, prop) => arr.reduce((prev, curr) => ((prev[curr[prop]] = ++prev[curr[prop]] || 1), prev), {});

TypeScript version

const countBy = <T extends Record<string, string>, K extends keyof T>(arr: T[], prop: K): Record<string, number> => (
arr.reduce((prev, curr) => ((prev[curr[prop]] = ++prev[curr[prop]] || 1), prev), {} as Record<string, number>)
);

Example

countBy(
[
{ 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': 2, 'ford': 2, 'bmw': 1 }
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