Had issue's properly solving this so I spent some time going over it.

duplicate-element-checker-as-module

//two seperate arrays with some duplicate numbers
//use object to hold values from larger array as keys
//compare second/smaller array against object.keys from first/larger array
//add duplicates to new array and return new array

var ArrayDuplicateFinder = (function() {

    //Private variable
    var arrayHash = {};

    var module = {
        run : run,
        found : { data: null },
        clear : clearData
    };

    return module;

    function run(first, second) {

        if (!Array.isArray(first) || !Array.isArray(second)) {
            return console.log('Not valid Array or Arrays');
        }

        //empty if array has existing elements
        if(module.found.data) clearData();

        checkArraySize(first, second, function(largeArray, smallArray) {
            mapObjectTo(largeArray, function() {
                compareWith(smallArray, function(dups) {
                    module.found.data = dups;
                });
            });
        });
    }

    function clearData() {
        module.found.data.length = 0;
        arrayHash = {};
    }

    function checkArraySize(first, second, callback) {

        //Note: Equal length arrays will work the same when mapped to object since same elements will be
        // checked against object key
        if (first.length == second.length) return callback(first, second);

        //We want larger array for mapping keys to avoid missing elements from smaller array
        var larger = first.length > second.length ? first : second;
        var smaller = first.length < second.length ? first : second;

        callback(larger, smaller);
    }

    function mapObjectTo(largerArray, done) {
        for (var i=0; i < largerArray.length; i++) {
            arrayHash[largerArray[i]] = largerArray[i];
        }
        return done();
    }

    function compareWith(small, callback) {
        var dups = [];
        small.forEach(function(el) {
            if (arrayHash.hasOwnProperty(el)) {
                dups.push(el);
            }
        });

        callback(dups);
    }

}());

var arrayA = [2, 5, 4, 6, 7, 10, 1],
    arrayB = [4, 8, 1, 11, 15, 9, 22];

var ADF = ArrayDuplicateFinder;

ADF.run(arrayA, arrayB);
//ADF.run(arrayB, arrayA);

var results = ADF.found.data;
console.log(results);
//alert(results);