Count the occurrences of array elements

JavaScript

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

TypeScript

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

Examples

countOccurrences([2, 1, 3, 3, 2, 3]) // { '1': 1, '2': 2, '3': 3 }
countOccurrences(['a', 'b', 'a', 'c', 'a', 'b']) // { 'a': 3, 'b': 2, 'c': 1 }