Shorten a string that contains emoticons.

unicodeSubstring.ts

function insertAt(string, subString, position) {
  const before = string.substr(0, position);
  const after = string.substr(position);
  return `${before}${subString}${after}`;
}

/**
 * Crude, but reliable way to do a substring with unicode chars. We tried other
 * solutions but they don't work with the flags which are 4 code points!
 */
export default function unicodeSubstring(
  string: string,
  start: number,
  end: number
): string {
  const unicodeRegex = /[^\x00-\x7F]+/g; // Plus is important!
  // Find all the unicode characters
  const matches = [...string.matchAll(unicodeRegex)];
  // Remove them all
  const without = string.replace(unicodeRegex, '');
  // Do the trim
  const trimmed = without.substr(start, end);
  // Re-insert the unicode characters which are in the substring
  let withUnicode = trimmed;
  for (let match of matches) {
    const position = match.index ?? 0;
    if (position >= start && position <= end) {
      withUnicode = insertAt(withUnicode, match[0], match.index);
    }
  }
  return withUnicode;
}