app.js

const buttons = Array.from(document.querySelectorAll("div > button"))
const numericButtons = buttons.filter(button => button.innerText.match(/\d/g))
const operatorsButtons = buttons.filter(button => button.innerText.match(/\+|\-|·|÷/g))
const decimalButton = buttons.filter(button => button.innerText == ",")[0]
const clearButton = buttons.filter(button => button.innerText == "C")[0]
const numberInput = document.querySelector("input[name=\"number\"]")
const operatorInput = document.querySelector("input[name=\"operator\"]")
const operationInput = document.querySelector("input[name=\"operation\"]")
const calculateInput = document.querySelector("input[name=\"calculate\"]")
const submitButton = document.querySelector("button[type=\"submit\"]")
const form = document.querySelector("form[name=\"calculate\"]")

let nextDecimal = false

operatorsButtons.forEach(
	operators => operators.addEventListener(
		"mouseup",
		() => {
			operationInput.value += operatorInput.value + numberInput.value
			operatorInput.value = operators.value
			form.submit()
		}
	)
)

numericButtons.forEach(
	number => number.addEventListener(
		"mouseup",
		() => {
			const numberValue = number.value

			if (nextDecimal) numberInput.value += ".".concat(numberValue)
			else if (numberInput.value === "0") numberInput.value = numberValue
			else numberInput.value += numberValue
				
			nextDecimal = false

			limitInput()
		}
	)
)

decimalButton.addEventListener(
	"mouseup",
	() => nextDecimal = numberInput.value.includes(".")? false:true
)

clearButton.addEventListener(
	"mouseup",
	() => numberInput.value = "0"
)

numberInput.addEventListener("input", limitInput)

numberInput.addEventListener("change", limitInput)

submitButton.addEventListener(
	"mousedown",
	() => {
		operationInput.value += operatorInput.value + numberInput.value
		calculateInput.value = "true"
		form.submit()
	}
)

function limitInput() {
	if (numberInput.value.length > 13) numberInput.value = numberInput.value.slice(0, 13)
}