The idea is to write a function that takes two strings and returns a new string that repeats in the previous two: examples:

 'ABBA' & 'AOHB' => 'AB'
 'cohs' & 'ohba' => 'oh'


A brute force solution would be nested for loops like so:

<!-- begin snippet: js hide: false console: true babel: false -->

<!-- language: lang-js -->

 const x = 'ABCD'
 const y = 'AHOB'

 function subStr(str1, str2) {
 let final = ''

 for (let i = 0; i < str1.length; i++) {
 for (let j = 0; j < str2.length; j++) {
 if (str1[i] === str2[j]) {
 final += str1[i]
 }
 }
 }

 return final
 }



 console.log(subStr(x, y)) // => AB

<!-- end snippet -->