replaceText = function(string, oldText, newText) {
  index = string.indexOf(oldText);
  front = string.substring(0, index);
  back = string.substring(index + oldText.length);
  return front + newText + back;
};

replaceTextGlobal = function(string, oldText, newText) {
  while(string.indexOf(oldText) >= 0) {
    string = replaceText(string, oldText, newText);	
  }
  return string;
};

console.log(replaceText("Randall","a","e"));
console.log(replaceTextGlobal("Randall","a","e"));
console.log(replaceText("Randall Reed","Randall","Randy"));