count the number of instances of items in an array code example

Example 1: javascript Count the frequency of a value in an array

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

// 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 }

Example 2: Count frequency of array elements js

var arr = [5, 5, 5, 2, 2, 2, 2, 2, 9, 4]

const map = arr.reduce((acc, e) => acc.set(e, (acc.get(e) || 0) + 1), new Map());

console.info([...map.keys()]) // to get unique elements
console.info([...map.values()]) // to get the occurrences
console.info([...map.entries()]) // to get the pairs [element, frequency]