var s = "some_string_to_change",
  len= s.length,
  i,
  upped = false;
  result = "";

for(i=0;i<len;i++) {
  if(s[i] === "_") {
    result += s[i+1].toUpperCase();
    upped = true;
  } else if (upped === false) {
    result += s[i];
  } else {
    upped = false;
  }
}