JSFiddle - React, Tailwind, and code Playground

HTML

simple counter:
<button class="minus">-</button>
<button class="plus">+</button>

<hr>

simple calculator:
<input class="n1" type="number" />
<input class="n2" type="number" />
<button class="result">加/減/乘/除</button>

JavaScript

const minusButton = document.querySelector('.minus');
const plusButton = document.querySelector('.plus');
const resultButton = document.querySelector('.result');

let number = 0;

minusButton.onclick = () => {
  number--;
  alert(number);
}

plusButton.onclick = () => {
  number++;
  alert(number);
}

function result() {
  const n1Value = document.querySelector('.n1').value;
  const n2Value = document.querySelector('.n2').value;
	if (n1Value === "" || n2Value === "") return alert('請輸入數字');
  alert(+n1Value + +n2Value);
  alert(+n1Value - +n2Value);
  alert(+n1Value * +n2Value);
  alert(+n1Value / +n2Value);
}

resultButton.onclick = result;