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function detectArabic(string) {
  try  {
    arCount = string.match(/[\u0600-\u06FF]/g).length;
    // results are approximative because whitespace and some ranges of arabic characters are ignored.
    return((string.length / arCount) > .5);
  } catch(e) {
    return(false);
  }
}

// to use on HTML element "el":
// detectArabic(el.innerText)