3-arrays.js

'use strict';

const union = (s1, s2) => {
  const ds = s1.slice(0);
  for (let i = 0; i < s2.length; i++) {
    const item = s2[i];
    if (!ds.includes(item)) ds.push(item);
  }
  return ds;
};

const intersection = (s1, s2) => {
  const ds = [];
  for (let i = 0; i < s1.length; i++) {
    const item = s1[i];
    if (s2.includes(item)) ds.push(item);
  }
  return ds;
};

const difference = (s1, s2) => {
  const ds = [];
  for (let i = 0; i < s1.length; i++) {
    const item = s1[i];
    if (!s2.includes(item)) ds.push(item);
  }
  return ds;
};

const complement = (s1, s2) => difference(s2, s1);

// Usage

const cities1 = ['Beijing', 'Kiev'];
const cities2 = ['Kiev', 'London', 'Baghdad'];

const operations = [union, intersection, difference, complement];

const results = operations.map((operation) => ({
  [operation.name]: operation(cities1, cities2)
}));

console.dir({ cities1, cities2 });
console.dir(results);