String.fromCodePoint()

String.fromCodePoint(42); // "*"
String.fromCodePoint(65, 90); // "AZ"
String.fromCodePoint(0x404); // "\u0404"
String.fromCodePoint(0x2f804); // "\uD87E\uDC04"
String.fromCodePoint(194564); // "\uD87E\uDC04"
String.fromCodePoint(0x1d306, 0x61, 0x1d307); // "\uD834\uDF06a\uD834\uDF07"

String.fromCodePoint("_"); // RangeError
String.fromCodePoint(Infinity); // RangeError
String.fromCodePoint(-1); // RangeError
String.fromCodePoint(3.14); // RangeError
String.fromCodePoint(3e-2); // RangeError
String.fromCodePoint(NaN); // RangeError

// String.fromCharCode() alone cannot get the character at such a high code point
// The following, on the other hand, can return a 4-byte character as well as the
// usual 2-byte ones (i.e., it can return a single character which actually has
// a string length of 2 instead of 1!)
console.log(String.fromCodePoint(0x2f804)); // or 194564 in decimal

/*! http://mths.be/fromcodepoint v0.1.0 by @mathias */
if (!String.fromCodePoint) {
  (function () {
    var defineProperty = (function () {
      // IE 8 only supports `Object.defineProperty` on DOM elements
      try {
        var object = {};
        var $defineProperty = Object.defineProperty;
        var result = $defineProperty(object, object, object) && $defineProperty;
      } catch (error) {}
      return result;
    })();
    var stringFromCharCode = String.fromCharCode;
    var floor = Math.floor;
    var fromCodePoint = function () {
      var MAX_SIZE = 0x4000;
      var codeUnits = [];
      var highSurrogate;
      var lowSurrogate;
      var index = -1;
      var length = arguments.length;
      if (!length) {
        return "";
      }
      var result = "";
      while (++index < length) {
        var codePoint = Number(arguments[index]);
        if (
          !isFinite(codePoint) || // `NaN`, `+Infinity`, or `-Infinity`
          codePoint < 0 || // not a valid Unicode code point
          codePoint > 0x10ffff || // not a valid Unicode code point
          floor(codePoint) != codePoint // not an integer
        ) {
          throw RangeError("Invalid code point: " + codePoint);
        }
        if (codePoint <= 0xffff) {
          // BMP code point
          codeUnits.push(codePoint);
        } else {
          // Astral code point; split in surrogate halves
          // http://mathiasbynens.be/notes/javascript-encoding#surrogate-formulae
          codePoint -= 0x10000;
          highSurrogate = (codePoint >> 10) + 0xd800;
          lowSurrogate = (codePoint % 0x400) + 0xdc00;
          codeUnits.push(highSurrogate, lowSurrogate);
        }
        if (index + 1 == length || codeUnits.length > MAX_SIZE) {
          result += stringFromCharCode.apply(null, codeUnits);
          codeUnits.length = 0;
        }
      }
      return result;
    };
    if (defineProperty) {
      defineProperty(String, "fromCodePoint", {
        value: fromCodePoint,
        configurable: true,
        writable: true,
      });
    } else {
      String.fromCodePoint = fromCodePoint;
    }
  })();
}