JSFiddle - React, Tailwind, and code Playground

HTML

<div class="plus">+</div>

<div class="res" ></div>

<div class="minus">-</div>

CSS

div{
  display:inline-block;
}

JavaScript

var number = 0;

function buybalance(){
	number = 2000;
	var res = document.querySelector('.res');
  res.innerHTML = number;
  res.addEventListener('click', buy);
}

function buy(){
	number -= 500;
	this.innerHTML = number;
}

buybalance();

function plu(){

	var get= document.querySelector('.plus');
	get.addEventListener('click',pluLis); 

};

function pluLis(){
	number += 500;
	var res = document.querySelector('.res');
	res.innerHTML = number;
};

plu();