1const countOccurrences = (arr, val) => arr.reduce((a, v) => (v === val ? a + 1 : a), 0);
2
3// Examples
4countOccurrences([2, 1, 3, 3, 2, 3], 2);                // 2
5countOccurrences(['a', 'b', 'a', 'c', 'a', 'b'], 'a');  // 3

1Array.prototype.frequencies = function() {
2    var l = this.length, result = {all:[]};
3    while (l--){
4       result[this[l]] = result[this[l]] ? ++result[this[l]] : 1;
5    }
6    // all pairs (label, frequencies) to an array of arrays(2)
7    for (var l in result){
8       if (result.hasOwnProperty(l) && l !== 'all'){
9          result.all.push([ l,result[l] ]);
10       }
11    }
12    return result;
13};
14
15var freqs = [5, 5, 5, 2, 2, 2, 2, 2, 9, 4].frequencies();
16alert(freqs[2]); //=> 5
17// or
18var freqs = '1,1,2,one,one,2,2,22,three,four,five,three,three,five'
19             .split(',')
20             .frequencies();
21alert(freqs.three); //=> 3