JSFiddle - React, Tailwind, and code Playground

HTML

<button id="btn1">-</button><input type="text" id="txt" /><button id="btn2">+</button>

JavaScript

window.onload=function(){
    var sumar=function(cantidad,txt){
        txt.value=(!isNaN(txt.value) && txt.value!="")?parseInt(txt.value)+cantidad:0;
    }
    var SUMADOR1={
        btnResta: document.getElementById("btn1"),
        btnSuma: document.getElementById("btn2"),
        txt: document.getElementById("txt")
    };
    SUMADOR1.btnResta.onclick=function(){ sumar(-1,SUMADOR1.txt); };
    SUMADOR1.btnSuma.onclick=function(){ sumar(1,SUMADOR1.txt); };
}