Counting the occurrences / frequency of array elements
You can use an object to hold the results:
const arr = [5, 5, 5, 2, 2, 2, 2, 2, 9, 4];const counts = {};for (const num of arr) { counts[num] = counts[num] ? counts[num] + 1 : 1;}console.log(counts[5], counts[2], counts[9], counts[4]);So, now your counts object can tell you what the count is for a particular number:
console.log(counts[5]); // logs '3'If you want to get an array of members, just use the keys() functions
keys(counts); // returns ["5", "2", "9", "4"]
const occurrences = [5, 5, 5, 2, 2, 2, 2, 2, 9, 4].reduce(function (acc, curr) { return acc[curr] ? ++acc[curr] : acc[curr] = 1, acc}, {});console.log(occurrences) // => {2: 5, 4: 1, 5: 3, 9: 1}
const arr = [2, 2, 5, 2, 2, 2, 4, 5, 5, 9];function foo (array) { let a = [], b = [], arr = [...array], // clone array so we don't change the original when using .sort() prev; arr.sort(); for (let element of arr) { if (element !== prev) { a.push(element); b.push(1); } else ++b[b.length - 1]; prev = element; } return [a, b];}const result = foo(arr);console.log('[' + result[0] + ']','[' + result[1] + ']')console.log(arr)