function replaceAt(str, index, ch) {
  return str.substr(0, index) + ch + str.substr(index + 1);
}
function replaceAt(str, index, ch) {
  return str.substr(0, index) + ch + str.substr(index + 1);
}
function replaceAt ( str , index , ch ) { return str . substr ( 0 , index ) + ch + str . substr ( index + 1 ) ; }
/* ’
/ * ’
Replace a character at a given index in a string