JSFiddle - React, Tailwind, and code Playground

by pierian_design

JavaScript

const str = 'ХуЙ';

function changeRegister (str) { 

let UPPER = 'АБВГДЕЁЖЗИЙКЛМНОПРСТУФХЦЧШЩЪЫЬЭЮЯ';
let LOWER = 'абвгдеёжзийклмнопрстуфхцчшщъыьэюя';
let result = [];
  
  for(var i=0; i<str.length; i++) {
    if(UPPER.includes(str[i])) {
      result.push(str[i].toLowerCase());
    }
    else if(LOWER.includes(str[i])) {
      result.push(str[i].toUpperCase());
    }
    else {
      result.push(str[i]);
    }
  }
  console.log(result);
  return result.join(''); 

}

console.log(changeRegister(str)); // [кАжДыЙ оХоТнИк]