Find Duplicates using reduce

find_duplicates.js

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

var arr = arr.reduce(function(acc,curr){
       acc[curr] ? acc[curr]++ : acc[curr]=1
       return acc
},{})

console.log(arr);

var str = "ABRACADABRA",
    obj = {}

for(var i = 0; i < str.length; ++i) {

    obj[str[i]] ? obj[str[i]]++ : obj[str[i]] = 1    
} 

console.log(obj);