let str = 'aAaA';

const makeLowerCase = (string) => {
  for ( var i = 0; i < string.length; i++ ) {
    let lower = string[i].toLowerCase();
    if (string[i] !== lower ) {
      string[i] = lower;
    }
  }
}

console.log('before', str);
makeLowerCase(str);
console.log('after', str);