Замена букв нижнего регистра на верхний и наоборот

let string  = 'hello MY name is JavaScript and I like your code styLe'; 
let result = replaceLetter(string);
 
function replaceLetter(item) {
    let letter = item.toUpperCase().split('');
    for (let i = 0; i < letter.length; i++) {
      if (letter[i] == item[i])
        letter[i] = letter[i].toLowerCase();
    }
    return letter.join("");
  }
 
  console.log(result);

// to console
`hello MY name is JavaScript and I like your code styLe`.replace(/[A-ZА-Я]+/gm, match => match.toLowerCase());

console.log(
  "hello MY name is JavaScript and I like your code styLe".replace(
    /[\S]*/g,
    (w) =>
      /^[A-Z]/.test(w)
        ? w
            .split("")
            .map((l) =>
              l.toUpperCase() == l ? l.toLowerCase() : l.toUpperCase()
            )
            .join("")
        : w
  )
);

let string  = 'hello MY name is JavaScript and I like your code styLe'; 
string.split(' ').map(word => word.match(reg) ? char(word) : word).join(' ');

let string = ('hello MY name is JavaScript and I like your code styLe.');
 
let result = string.replace(/(([A-Za-z])+([a-z]))*([a-z]*[A-Z]+[a-z]*)/g, function replaceLetter (letter) {
  if(letter === letter.toUpperCase()) {
    for(let i=0; i = letter.toUpperCase(); i++){
      letter = letter.toLowerCase();
    }
    return letter;
  }
  
  else {
    for(let i=0; i = letter.toLowerCase(); i++) {
      letter = letter.toUpperCase();
    }
    return letter;
  }
})
 
console.log(result);

  /[A-Z]/.test(w)

let string  = 'hello MY name is JavaScript and I like your code styLe'; 
string.split(' ').map(word => word.match(reg) ? char(word) : word).join(' ');

let string = ('hello MY name is JavaScript and I like your code styLe.');
 
let result = string.replace(/(([A-Za-z])+([a-z]))*([a-z]*[A-Z]+[a-z]*)/g, function replaceLetter (letter) {
  if(letter === letter.toUpperCase()) {
    for(let i=0; i = letter.toUpperCase(); i++){
      letter = letter.toLowerCase();
    }
    return letter;
  }
  
  else {
    for(let i=0; i = letter.toLowerCase(); i++) {
      letter = letter.toUpperCase();
    }
    return letter;
  }
})
 
console.log(result);

        function replaceLetter(str) {
            let result = "";
            for (let c of str)
                result += c === c.toUpperCase() ? c.toLowerCase() : c.toUpperCase();
 
            return result;
        }

        const replaceLetter = str => [...str].reduce(
            (r, c) => r + (c === c.toUpperCase() ? c.toLowerCase() : c.toUpperCase()), "");
            
        let string = 'hello MY name is JavaScript and I like your code styLe';
        let result = replaceLetter(string);
        console.log(result);

        function replaceLetter(str) {
            let result = "";
            for (let i = 0; i < str.length; i++) {
                let c = str[i];
                result += c === c.toUpperCase() ? c.toLowerCase() : c.toUpperCase();
            }
            return result;
        }

        const replaceLetter = str => [...str].map(
            c => c === c.toUpperCase() ? c.toLowerCase() : c.toUpperCase()).join("");

        const replaceLetter = str => str.replace(/\S+/g,
            m => [...m].some(c => c === c.toUpperCase()) ? innerReplace(m) : m);

        const replaceLetter = str => str.replace(/\S+/g,
            m => /\p{Lu}/u.test(m) ? innerReplace(m) : m);