Edit in JSFiddle

let firstRecurringCharacter = string => {
	let array = Array.from(string); 
	let recurringCharacter = array.find((character, index) => {
  	return array.indexOf(character) < index;
  });
  
   let output =
     recurringCharacter ? recurringCharacter : "No recurring character";
  
  /**
   *	Using document.write so you can see the output
   *  without opening the console ;)
   */
  document.write(output + "<br>");
}

firstRecurringCharacter('ABCA');
firstRecurringCharacter('BCABA');
firstRecurringCharacter('ABC');