How about an ECMAScript2015 option.
const a = [5, 5, 5, 2, 2, 2, 2, 2, 9, 4];
const aCount = new Map([...new Set(a)].map(
x => [x, a.filter(y => y === x).length]
));
aCount.get(5) // 3
aCount.get(2) // 5
aCount.get(9) // 1
aCount.get(4) // 1
This example passes the input array to the Set constructor creating a collection of unique values. The spread syntax then expands these values into a new array so we can call map and translate this into a two-dimensional array of [value, count] pairs - i.e. the following structure:
Array [
[5, 3],
[2, 5],
[9, 1],
[4, 1]
]
The new array is then passed to the Map constructor resulting in an iterable object:
Map {
5 => 3,
2 => 5,
9 => 1,
4 => 1
}
The great thing about a Map object is that it preserves data-types - that is to say aCount.get(5) will return 3 but aCount.get("5") will return undefined. It also allows for any value / type to act as a key meaning this solution will also work with an array of objects.
function frequencies(/* {Array} */ a){_x000D_
return new Map([...new Set(a)].map(_x000D_
x => [x, a.filter(y => y === x).length]_x000D_
));_x000D_
}_x000D_
_x000D_
let foo = { value: 'foo' },_x000D_
bar = { value: 'bar' },_x000D_
baz = { value: 'baz' };_x000D_
_x000D_
let aNumbers = [5, 5, 5, 2, 2, 2, 2, 2, 9, 4],_x000D_
aObjects = [foo, bar, foo, foo, baz, bar];_x000D_
_x000D_
frequencies(aNumbers).forEach((val, key) => console.log(key + ': ' + val));_x000D_
frequencies(aObjects).forEach((val, key) => console.log(key.value + ': ' + val));