Use reduce to count the occurrence of an item in a list.

reduce-count.js

/*
    Using reduce to count the occurrence of an item in a list.

    [ 'd', 'a', 'b', 'a', 'b', 'c', 'd', 'b', 'd', 'd' ]
    a occurs 2 times in the list.
    b occurs 3 times in the list.
    c occurs 1 time in the list.
    d occurs 4 times in the list.
 */

const list = [ 'd', 'a', 'b', 'a', 'b', 'c', 'd', 'b', 'd', 'd']

function countTimes( item ) {

    const count = list.reduce( (prev, curr) => prev += curr == item ? 1 : 0, 0);

    console.log( `${item} occurs ${count} time${ count == 1 ? '' : 's'} in the list.` );
}

console.log(list);

countTimes('a');
countTimes('b');
countTimes('c');
countTimes('d');