code: function isSurrogatePair(str, index) {
    invariant(
      0 <= index && index < str.length,
      'isSurrogatePair: Invalid index %s for string length %s.',
      index,
      str.length
    );
    if (index + 1 === str.length) {
      return false;
    }
    const first = str.charCodeAt(index);
    const second = str.charCodeAt(index + 1);
    return (
      SURROGATE_HIGH_START <= first && first <= SURROGATE_HIGH_END &&
      SURROGATE_LOW_START <= second && second <= SURROGATE_LOW_END
    );
  }
docs: Returns whether the two characters starting at `index` form a surrogate pair.
  For example, given the string s = "\uD83D\uDE0A", (s, 0) returns true and
  (s, 1) returns false.
  
  @param {string} str
  @param {number} index
  @return {boolean}