arrayOfDuplicates.js

function duplicates(arr) {
    var mergedArray = [].concat.apply([], arr);
    console.log(mergedArray);

    var arrSorted = mergedArray.sort(function (a, b) {
        return a - b;
    });

    console.log(arrSorted);

    var arrayOfDuplicatesArray = arrSorted.reduce(function(acc, val) {
        var inner;
        
        if (acc.previous !== val) {
                inner = [];
            } else {
                inner = acc.newArray.pop();
            }
    
            inner.push(val);
            acc.previous = val;
            acc.newArray.push(inner);
    
            return acc;
        }, {
            previous: null,
            newArray: []
    });

    console.log(arrayOfDuplicatesArray.newArray);
    
    var temporaryArray = arrayOfDuplicatesArray.newArray;
    
    //console.log(temporaryArray)
    
    var arrayOfDuplicates = temporaryArray.filter(function (currElem) {
        return currElem.length > 1;
    }).map(function (currElem) {
        return currElem[0];
    })
    
    return arrayOfDuplicates;
    //console.log(arrayOfDuplicates);
}

duplicates([0, 1, 2, 4, 4, 3, 3, 1, 5, 3, 3, '5', '5', [2, 2]]);