count occurrences in object list javascript code example

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

const countOccurrences = (arr, val) => arr.reduce((a, v) => (v === val ? a + 1 : a), 0);

// Examples
countOccurrences([2, 1, 3, 3, 2, 3], 2);                // 2
countOccurrences(['a', 'b', 'a', 'c', 'a', 'b'], 'a');  // 3

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]