docs: function getNewChars(key, characterSet) {
    const cachedFontAtlas = cache.get(key);
    if (!cachedFontAtlas) {
      return characterSet;
    }
  
    const newChars = [];
    const cachedMapping = cachedFontAtlas.mapping;
    let cachedCharSet = Object.keys(cachedMapping);
    cachedCharSet = new Set(cachedCharSet);
  
    let charSet = characterSet;
    if (charSet instanceof Array) {
      charSet = new Set(charSet);
    }
  
    charSet.forEach(char => {
      if (!cachedCharSet.has(char)) {
        newChars.push(char);
      }
    });
  
    return newChars;
  }