stringFromCodePoint.js

const _String = String

export default _String.fromCodePoint || function stringFromCodePoint() {
  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
      Math.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 += String.fromCharCode.apply(null, codeUnits);
      codeUnits.length = 0;
    }
  }
  return result;
};