Javascript – im making a calculator and facing some unknown issues

So I decided to make a calculator for practice sake, and now I’m facing the issue in which console doesn’t say any errors, it shows digits on screen, but it doesn’t calculate them at all, just adds these values 1 by 1. Here’s the code:

let buffer = '0';
let runningTotal = 0;
let previousOperator;


const screen = document.querySelector('.screen');
const all_buttons = document.querySelectorAll('.calc_btns .calc_btn');

function buttonClick(value){
    if(isNaN(value)){
        handleButtonClick(value);
    }
    else{
        handleNumber(value);
    }
    updateScreen();
}

function handleButtonClick(symbol){
    switch(symbol){
        case 'A':
            clearBuffer();
            break;
        case '=':
            if(previousOperator == null){
                return;
            }
            mathOperator(parseFloat(buffer));
            buffer = runningTotal.toString();
            runningTotal = 0;
            break;
        case '%':
        case '-':
        case '*':
        case '÷':
        case '×':
            handleMath(symbol);
            break;
        default:
            handleNumber(symbol);
            break;
    }
    updateScreen();
}

function init(){
    for(let i = 0; i < all_buttons.length; i++){
        let button = all_buttons[i];
        button.addEventListener('click', function (){
            handleButtonClick(this.textContent);
            console.log('button clicked');
        })
    }
}
init();

function handleMath(symbol){

    if(buffer == '0'){
        return;
    }

    const intBuffer = parseFloat(buffer);

    if(runningTotal == 0){
        runningTotal = intBuffer;
    }
    else{
        handleButtonClick(intBuffer);
    }
    mathOperator(symbol);
    buffer = '0';
}
function mathOperator(intBuffer){
    if(previousOperator == '+'){
        runningTotal += intBuffer;
    }
    else if(previousOperator == '-'){
        runningTotal -= intBuffer;
    }
    else if(previousOperator == '%'){
        runningTotal %= intBuffer;
    }
    else if(previousOperator == '÷'){
        runningTotal /= intBuffer;
    }
    else if(previousOperator == '×'){
        runningTotal *= intBuffer;
    }
}

function handleNumber(numberString){
    if(buffer == '0'){
        buffer = numberString;
    }
    else{
        buffer += numberString;
    }
}

function clearBuffer(){
    buffer = '0';
    runningTotal = 0;
    previousOperator = undefined;
}

function updateScreen(){
    screen.textContent = buffer;
}

body {
  background-color: grey;
}

.container {
  margin-left: 50% auto;
  margin-top: 100px;
  border: 3px solid;
  border-radius: 15px;
  padding: 25px 30px;
  width: 240px;
  height: 360px;
  font-size: 40px;
  font-family: "Ubuntu", sans-serif;
  background-color: black;
  color: white;
  border-color: black;
  box-shadow: 15px 15px 20px black;
}

.calc_btn {
  border-radius: 50px;
  border: 1px solid;
  font-family: "Ubuntu", sans-serif;
  font-size: 30px;
  padding: 5px 9px;
  width: 50px;
  height: 50px;
  background-color: rgb(71, 69, 69);
  color: white;
  border-color: rgb(71, 69, 69);
  display: inline;
}

.calc_btn:hover {
  background-color: grey;
}

.calc_btn:last-child {
  background-color: orange;
}

.calc_btn:last-child:hover {
  background-color: rgb(235, 179, 76);
}

.screen {
  text-align: right;
  padding: 5px;
  font-family: inherit;
  font-size: 45px;
  border: 1px solid;
  border-radius: 15px;
}

.calc_btn_row {
  display: flex;
  justify-content: space-between;
  margin: 5% 0;
}

<div class="container">
  <section class="screen">
    0
  </section>

  <section class="calc_btns">
    <div class="calc_btn_row">
      <button class="calc_btn">
            A
          </button>
      <button class="calc_btn">
            /-
          </button>
      <button class="calc_btn">
            %
          </button>
      <button class="calc_btn">
            ÷
          </button>
    </div>
    <div class="calc_btn_row">
      <button class="calc_btn">
            7
          </button>
      <button class="calc_btn">
            8
          </button>
      <button class="calc_btn">
            9
          </button>
      <button class="calc_btn">
            ×
          </button>
    </div>
    <div class="calc_btn_row">
      <button class="calc_btn">
            4
          </button>
      <button class="calc_btn">
            5
          </button>
      <button class="calc_btn">
            6
          </button>
      <button class="calc_btn">
            -
          </button>
    </div>
    <div class="calc_btn_row">
      <button class="calc_btn">
            1
          </button>
      <button class="calc_btn">
            2
          </button>
      <button class="calc_btn">
            3
          </button>
      <button class="calc_btn">
            +
          </button>
    </div>
    <div class="calc_btn_row">
      <button id="num0" class="calc_btn">
            0
          </button>
      <button class="calc_btn">
            ,
          </button>
      <button class="calc_btn">
            =
          </button>
    </div>
  </section>
</div>

I also have an updated video.