Count by the properties of an array of objects

JavaScript

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

TypeScript

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 }