letterreplace.js

var str = "aaaaaaaa";

var word = "aaa";

var ltr = "w";

var result = "";

for(var i = 0; i < str.length; i++) {
  //console.log(str)
  //console.log('1 ' +  i + ' ' + ' ' + str.substring(i,i+word.length));
  if(word == str.substring(i,i+word.length)) {
    result += ltr;
    i = i+word.length-1;
  } else {
    console.log('else');
    result+=str[i];
  }
  //console.log('1 ' +  i + ' ' + result)
  console.log(i)
}

console.log('result', result)