string.charCodeAt

str.charCodeAt(index)

'ABC'.charCodeAt(0 // returns 65

function fixedCharCodeAt(str, idx) {
  // ex. fixedCharCodeAt('\uD800\uDC00', 0 // 65536
  // ex. fixedCharCodeAt('\uD800\uDC00', 1 // false
  idx = idx || 0;
  var code = str.charCodeAt(idx
  var hi, low;
  
  // High surrogate (could change last hex to 0xDB7F
  // to treat high private surrogates 
  // as single characters)
  if (0xD800 <= code && code <= 0xDBFF) {
    hi = code;
    low = str.charCodeAt(idx + 1
    if (isNaN(low)) {
      throw 'High surrogate not followed by ' +
        'low surrogate in fixedCharCodeAt()';
    }
    return ((hi - 0xD800) * 0x400) +
      (low - 0xDC00) + 0x10000;
  }
  if (0xDC00 <= code && code <= 0xDFFF) { // Low surrogate
    // We return false to allow loops to skip
    // this iteration since should have already handled
    // high surrogate above in the previous iteration
    return false;
    // hi = str.charCodeAt(idx - 1
    // low = code;
    // return ((hi - 0xD800) * 0x400) +
    //   (low - 0xDC00) + 0x10000;
  }
  return code;
}

function knownCharCodeAt(str, idx) {
  str += '';
  var code,
      end = str.length;

  var surrogatePairs = /[\uD800-\uDBFF][\uDC00-\uDFFF]/g;
  while ((surrogatePairs.exec(str)) != null) {
    var li = surrogatePairs.lastIndex;
    if (li - 2 < idx) {
      idx++;
    }
    else {
      break;
    }
  }

  if (idx >= end || idx < 0) {
    return NaN;
  }

  code = str.charCodeAt(idx

  var hi, low;
  if (0xD800 <= code && code <= 0xDBFF) {
    hi = code;
    low = str.charCodeAt(idx + 1
    // Go one further, since one of the "characters"
    // is part of a surrogate pair
    return ((hi - 0xD800) * 0x400) +
      (low - 0xDC00) + 0x10000;
  }
  return code;
}