JSFiddle - React, Tailwind, and code Playground

HTML

<textarea name="txt" id="ta" cols="30" rows="10"></textarea>

JavaScript

var persianLetters = ['آ ', 'ا', 'ب', 'پ', 'ت', 'ث','ج', 'چ', 'ح', 'خ', 'د', 'ذ', 'ر', 'ز' , 'ژ', 'س', 'ش', 'ص', 'ض', 'ط', 'ظ', 'ع', 'غ', 'ف', 'ق', 'ک', 'گ' ,'ل', 'م', 'ن', 'و', 'ه', 'ى'];

$("#ta").on('input', function() {
  
      var el  = $(this);
      var txt = el.val();
      var len = txt.trim().length;
  
      if (len <= 1){
	    var x = txt.substring(0, 1);
      
      console.log($.inArray(x, persianLetters));
	    if($.inArray(x, persianLetters) > -1){
      	el.css("direction", "rtl");
      } else {
      	el.css("direction", "ltr");
      }
    }
 
  
});