JSFiddle - React, Tailwind, and code Playground

HTML

simple counter:
      <button id="minus_btn" onclick="minus_btn()">-</button>
      <span id="rz">0</span>
      <button id="plus_btn" onclick="plus_btn()">+</button>

      <hr>

      simple calculator:
      <input type="text" id="x_in"/>
      (+ / − / × / ÷)
      <input type="text" id="y_in" />
      <button id="operation_btn" onclick="operation()">加/減/乘/除</button>

JavaScript

var rz = 0;

const minus_btn = () => {

    rz = rz - 1;
    alert(rz);
    document.querySelector('#rz').textContent = rz;
}

const plus_btn = () => {

    rz = rz + 1;
    alert(rz);
    document.querySelector('#rz').textContent = rz;
}

const operation = () => {

    const x_in = parseInt(document.querySelector('#x_in').value);
    const y_in = parseInt(document.querySelector('#y_in').value);

    alert("x_in + y_in = " + (x_in + y_in));
    alert("x_in - y_in = " + (x_in - y_in));
    alert("x_in × y_in = " + (x_in * y_in));
    alert("x_in ÷ y_in = " + (x_in / y_in));
}