String.prototype.codePointAt()

"ABC".codePointAt(1); // 66
"\uD800\uDC00".codePointAt(0); // 65536

"XYZ".codePointAt(42); // undefined

/*! http://mths.be/codepointat v0.1.0  @mathias */
if (!String.prototype.codePointAt) {
  (function () {
    "use strict"; //     `apply`/`call`  `undefined`/`null`
    var codePointAt = function (position) {
      if (this == null) {
        throw TypeError();
      }
      var string = String(this);
      var size = string.length;
      // `ToInteger`
      var index = position ? Number(position) : 0;
      if (index != index) {
        // ,  `isNaN`
        index = 0;
      }
      //      
      if (index < 0 || index >= size) {
        return undefined;
      }
      //    
      var first = string.charCodeAt(index);
      var second;
      if (
        // ,      
        first >= 0xd800 &&
        first <= 0xdbff && //    
        size > index + 1 //   
      ) {
        second = string.charCodeAt(index + 1);
        if (second >= 0xdc00 && second <= 0xdfff) {
          //    
          // http://mathiasbynens.be/notes/javascript-encoding#surrogate-formulae
          return (first - 0xd800) * 0x400 + second - 0xdc00 + 0x10000;
        }
      }
      return first;
    };
    if (Object.defineProperty) {
      Object.defineProperty(String.prototype, "codePointAt", {
        value: codePointAt,
        configurable: true,
        writable: true,
      });
    } else {
      String.prototype.codePointAt = codePointAt;
    }
  })();
}