How to rebuild this code for OpenCart3?

Question

JavaScript

Good day, I want to redo the code, I have a code that changes the HTML values, and I need it to change the VALUE in the input, now I will send the standard OpenCart3 code and mine)
There is nothing like that just to adjust

<input type="text" name="quantity[{{ product.cart_id }}]" value="{{ product.quantity }}" size="1" class="form-control" />

<div class="row counter">
                            <input type="button" class="remove-counter" id="buttonCountMinus" value="-">
                            <div id="buttonCountNumber">1</div>
                            <input type="button" class="add-counter" id="buttonCountPlus" value="+">
                        </div>
<div id="price" style="opacity: 0;">350</div>
                          <div id="calculation" class="price-cart cart-counter">350</div>
                          <div class="row price-count" style="color: #d70506;">
                            <div class="price-counter">350 x </div>
                            <input type="text" style="width: 20%; border: none; background: none; position: relative; left: 3px; color: #d70506;" value="1" id="num" class="cart-counter" name="num" readonly>
                          </div>
<div class="col-10 full-price-row" style="margin-top: 5%;">
                  <div class="row row-price">
                    <p class="start-price-text" style="font-weight: 400;">Итого:</p>
                      <div style="margin-right: 3%;" id="calculation-2" class="full-price">350 грн</div>
                  </div>
                </div>
                <div class="col-10 full-price-row">
                  <div class="row row-price" style="color: #d70506;">
                    <p class="full-price-text" style="font-weight: 400;">Сумма к оплате:</p>
                    <div style="margin-right: 3%; margin-top: 1%;" id="calculation-3" class="full-price">350 грн</div>
                  </div>
                </div>
      let calculate = document.getElementById("calculation");
let count = document.getElementById("buttonCountNumber");
let count2 = document.getElementById("num");
let number = 1;
calculation = document.getElementById("calculation").innerHTML;
let calculate2 = document.getElementById("calculation-2");
calculation2 = document.getElementById("calculation-2").innerHTML;
let calculate3 = document.getElementById("calculation-3");
calculation3 = document.getElementById("calculation-3").innerHTML;

document.getElementById("buttonCountPlus").onclick = function() {
  let countPlus = count.innerHTML;
  if(+countPlus <= 3){
    number++;
    count.innerHTML++;
    count2.value = number;
    let countPlus = count.innerHTML;
    calculate.innerHTML = calculations(countPlus) ;
    calculate2.innerHTML = calculations2(countPlus) ;
    calculate3.innerHTML = calculations3(countPlus) ;
  }
}

document.getElementById("buttonCountMinus").onclick = function() {
  let countMinus = count.innerHTML;
  if(+countMinus >= 2){
    number--;
    count.innerHTML--;
    count2.value = number;
    let countMinus = count.innerHTML;
    calculate.innerHTML = calculations(countMinus) ;
    calculate2.innerHTML = calculations2(countMinus) ;
    calculate3.innerHTML = calculations3(countMinus) ;
  }
}

calculations = (count) => {
  return  (+count)*(+calculation);
}
calculations2 = (count) => {
  return  (+count)*(+calculation) + ` ` + `грн`;
}

calculations3 = (count) => {
  return  (+count)*(+calculation) + ` ` + `грн`;
}

Here the code and HTML are involved, etc., but most of all I'm interested in changing the values ​​​​under VALUE, I threw off the rest of the code so that it would be convenient for you and not confuse you)

Answer 1

If something your code does not work, there are errors, but this is the lyrics.

<input type="number" id="calculation" value="350" />

//Читаем value у элемента выше
let calculation = document.getElementById("calculation").value;
console.log(calculation);
//меняем  value  вместо тех мест где .innerHTML
let calculation1 = document.getElementById("calculation").value = "234";

console.log(calculation1);