function stringChanger(str) {
    let result = ""
    let letters = ["а", "е", "ё", "и", "о", "у", "ы", "э", "ю", "я"]
    
    for (let i = 0; i < str.length; i += 1) {
        if (letters.includes(str[i].toLowerCase())) {
            result = result + str[i].toUpperCase()
        } else {
        result = result + str[i].toLowerCase()
        }
    }
    return result
}
  
console.log(stringChanger("Привет Мир")) // "прИвЕт мИр"
console.log(stringChanger("Азбука Вкуса")) // "АзбУкА вкУсА"

.canvas {
  background: var(--gradient);
  mask: var(--dots), var(--noise);
  mask-composite: intersect /* source-in, xor */;
  animation: flicker;
}
@keyframes flicker {
  to { mask-position: 50% 50% , 0 50%; }
}

mask: radial-gradient(circle at 50% 50%, white 2px, transparent 2.5px) 50% 50% / 40px 40px;

mask: url("noise.png") 50% 50% / 256px 256px;

input:valid { --valid: 1; }
input {
    background: var(--bg) padding-box, ...,
    var(--valid-bg) calc((1 - var(--valid)) * 100cqi) 0 /
    100% 100% border-box;
}

@media (prefers-reduced-motion: no-preference) {
    input { transition: background-position: 0.5s; }
}