JSFiddle - React, Tailwind, and code Playground

HTML

<input type = "text" class = 'firstN' placeholder="Введите число">  


<div class="radio-wrapper">
    <input type = "radio" id = "plus" name = "rad"><label> +</label>
    <input type = "radio" id = "minus" name = "rad"> <label>-</label>
    <input type = "radio" id = "mult" name = "rad"> <label>*</label>
</div>
<br/><br/>

<input type = "button" class = "key" id = "one" value = "1">
<input type = "button" class = "key" id = "two" value = "2">
<input type = "button" class = "key" id = "three" value = "3">
<br/><br/>
<input type = "button" class = "key" id = "four" value = "4">
<input type = "button" class = "key" id = "five" value = "5">
<input type = "button" class = "key" id = "six" value = "6">
<br/><br/>
<input type = "button" class = "key" id = "seven" value = "7">
<input type = "button" class = "key" id = "eight" value = "8">
<input type = "button" class = "key" id = "nine" value = "9">
<br/><br/>
<input type = "button" class = "key" id = "zero" value = "0">

<div class = "invisible"></div>
<br/><br/>


<input type = "button" class = "btn" value = "=">
<input type = "reset" class = "btn" value = "C">

CSS

.key {
    display:inline-block;
    background:#ccc;
    float:left;
    height:20px;
    width:20px;
}

.invisible {content:''; clear: both;}

JavaScript

var arr = [];
  

var onekey = document.querySelector('#one');
onekey.addEventListener('click', result, false);

var twokey = document.querySelector('#two');
twokey.addEventListener('click', result, false);

var threekey = document.querySelector('#three');
threekey.addEventListener('click', result, false);

var fourkey = document.querySelector('#four');
fourkey.addEventListener('click', result, false);

var fivekey = document.querySelector('#five');
fivekey.addEventListener('click', result, false);

var sixkey = document.querySelector('#six');
sixkey.addEventListener('click', result, false);

var sevenkey = document.querySelector('#seven');
sevenkey.addEventListener('click', result, false);

var eightkey = document.querySelector('#eight');
eightkey.addEventListener('click', result, false);

var ninekey = document.querySelector('#nine');
ninekey.addEventListener('click', result, false);

var zerokey = document.querySelector('#zero');
zerokey.addEventListener('click', result, false);

var itr =0
function result(event) {
    itr=itr+=1
   
    console.log(itr+' step: '+ arr)
      document.querySelector('.firstN').value = arr.join(' ');
    console.log(itr+' step: '+ arr)
      arr.push(event.target.value);
    console.log(itr+' step: '+ arr)

}