JSFiddle - React, Tailwind, and code Playground

HTML

<input type="text" id="card" placeholder="Введите номер карты"><span id="discount">нет скидки</span>

JavaScript

var cards = {
	'1111':'1',
  '2222':'2',
  '3333':'3',
  '4444':'4',
};
var card = document.getElementById('card');
var discount = document.getElementById('discount');
card.addEventListener('input', test_card, false);
function test_card()
{
	discount.innerHTML = 'нет скидки';
	if(card.value.length == 4)
  {
  	if(cards[card.value] != undefined) discount.innerHTML = 'имеет скидку '+cards[card.value]+'%';
  }
  
}