Annie_loop

var s = "some_string_to_change",
  len= s.length,
  i,
  upped = false; // we will use this to make sure we don't double count a character
  result = ""; // this is where we will store the result

for(i=0;i<len;i++) {
  if(s[i] === "_") { // if we find an underscore
    result += s[i+1].toUpperCase(); // add the next character, uppercased, to our result 
    upped = true; // make sure we don't add this character again
  } else if (upped === false) { // skip any characters we've added.
    result += s[i]; // add the current character to our result
  } else {
    upped = false; // reset the flag so we can carry on
  }
}

console.log(result) // output the result to the console.