function findCommonElem(arr1, arr2) {
const map = new Map();
arr1.forEach(item => map.set(item, true));
return arr2.filter(item => map.has(item));
}
const arr1 = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10];
const arr2 = [2, 4, 6, 8, 10, 12, 14, 16, 20];
console.log(findCommonElem(arr1, arr2)); // [2, 4, 6, 8, 10];