Counting the occurrences / frequency of array elements

Question

In Javascript, I\'m trying to take an initial array of number values and count the elements inside it. Ideally, the result would be two new arrays, the first specifying each

Answer 1

I think this is the simplest way how to count occurrences with same value in array.

var a = [true, false, false, false];
a.filter(function(value){
    return value === false;
}).length

Answer 2

One line ES6 solution. So many answers using object as a map but I can't see anyone using an actual Map

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

Use map.keys() to get unique elements

Use map.values() to get the occurrences

Use map.entries() to get the pairs [element, frequency]

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()])
console.info([...map.values()])
console.info([...map.entries()])

Answer 3

Here you go:

Live demo: http://jsfiddle.net/simevidas/bnACW/

Note

This changes the order of the original input array using Array.sort

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

function foo(arr) {
  var a = [],
    b = [],
    prev;

  arr.sort();
  for (var i = 0; i < arr.length; i++) {
    if (arr[i] !== prev) {
      a.push(arr[i]);
      b.push(1);
    } else {
      b[b.length - 1]++;
    }
    prev = arr[i];
  }

  return [a, b];
}

var result = foo(arr);
console.log('[' + result[0] + ']','[' + result[1] + ']')

Answer 4

var a = [5, 5, 5, 2, 2, 2, 2, 2, 9, 4].reduce(function (acc, curr) {
  if (typeof acc[curr] == 'undefined') {
    acc[curr] = 1;
  } else {
    acc[curr] += 1;
  }

  return acc;
}, {});

// a == {2: 5, 4: 1, 5: 3, 9: 1}

Answer 5

function countOcurrences(arr){
    return arr.reduce((aggregator, value, index, array) => {
      if(!aggregator[value]){
        return aggregator = {...aggregator, [value]: 1};  
      }else{
        return aggregator = {...aggregator, [value]:++aggregator[value]};
      }
    }, {})
}

Answer 6

ES6 solution with reduce (fixed):

const arr = [2, 2, 2, 3, 2]

const count = arr.reduce((pre, cur) => (cur === 2) ? ++pre : pre, 0)
console.log(count) // 4