JSFiddle - React, Tailwind, and code Playground

JavaScript

function arr_sim (a1, a2) {

    var a = [], sim = [];

    for (var i = 0; i < a2.length; i++) {
        a[a1[i]] = true;
    }

    for (var i = 0; i < a1.length; i++) {
        if (a[a2[i]]) {
            a[a2[i]] = true;
        } else {
            delete a[a1];
        }
    }

    for (var k in a) {
        sim.push(k);
    }

    return sim;
}

console.log(arr_sim(['a', 'b'], ['b', 'a', 'c', 'd']));
console.log(arr_sim("abcd", "abcde"));
console.log(arr_sim("cxz", "zcx"));