Find common elements between two arrays.

array-elements-in-common.js

Array.prototype.same = function(arr2) {
    let same = new Set();
    let arr1 = this;
    arr1.forEach(item => !!arr2.includes(item) && same.add(item));
    arr2.forEach(item => !!arr1.includes(item) && same.add(item));
    return Array.from(same);
};