5kyu_PlayWithTwoStrings.js

// 5kyu - Play with Two Strings 

// Your task is to Combine two Strings. But consider the rule...

// By the way you don't have to check errors or incorrect input values, everything is ok without bad tricks, only two input strings and as result one output string;-)...

// And here's the rule:
// Input Strings a and b: For every character in string a swap the casing of every occurrence of the same character in string b. Then do the same casing swap with the inputs reversed. Return a single string consisting of the changed version of a followed by the changed version of b. A char of a is in b regardless if it's in upper or lower case - see the testcases too.
// I think that's all;-)...

// Some easy examples:

// Input: "abc" and "cde"      => Output: "abCCde" 
// Input: "ab" and "aba"       => Output: "aBABA"
// Input: "abab" and "bababa"  => Output: "ABABbababa"
// Once again for the last example - description from KenKamau, see discourse;-):

// a) swap the case of characters in string b for every occurrence of that character in string a
// char 'a' occurs twice in string a, so we swap all 'a' in string b twice. This means we start with "bababa" then "bAbAbA" => "bababa"
// char 'b' occurs twice in string a and so string b moves as follows: start with "bababa" then "BaBaBa" => "bababa"

// b) then, swap the case of characters in string a for every occurrence in string b
// char 'a' occurs 3 times in string b. So string a swaps cases as follows: start with "abab" then => "AbAb" => "abab" => "AbAb"
// char 'b' occurs 3 times in string b. So string a swaps as follow: start with "AbAb" then => "ABAB" => "AbAb" => "ABAB".

// c) merge new strings a and b
// return "ABABbababa"

// There are some static tests at the beginning and many random tests if you submit your solution.

// Hope you have fun:-)!

function workOnStrings(a,b){
  let cc=x=>x==x.toLowerCase()?x.toUpperCase():x.toLowerCase(), arra=a.split(''), arrb=b.split(''), r, res;
  res =  arra.map(x=>{r=new RegExp(x,'gi'); return (b.match(r)||[]).length%2?cc(x):x}).join('');
  res += arrb.map(x=>{r=new RegExp(x,'gi'); return (a.match(r)||[]).length%2?cc(x):x}).join('');
  return res;
}