index.js

function numButtonPressed(num) {
  const screenDiv = document.getElementById("screen");

  if (screenDiv.innerHTML.includes("h1")) {
    screenDiv.innerHTML = num;
  } else {
    screenDiv.innerHTML = screenDiv.innerHTML + num;
  }
}

function equalButtonPressed() {
  const screenDiv = document.getElementById("screen");
  try {
    const result = eval(screenDiv.innerHTML);
    screenDiv.innerHTML = result;
  } catch (error) {
    console.error(error);
    const screenBeforeError = screenDiv.innerHTML;
    screenDiv.innerHTML = "ERROR";

    setTimeout(function () {
      screenDiv.innerHTML = screenBeforeError;
    }, 3000);
  }
}

function clearButtonPressed() {
  const screenDiv = document.getElementById("screen");
  if (screenDiv.innerHTML.length === 1) {
    screenDiv.innerHTML = 0;
  } else {
    screenDiv.innerHTML = screenDiv.innerHTML.slice(
      0,
      screenDiv.innerHTML.length - 1
    );
  }
}

function plusMinusButtonPressed() {
  const screenDiv = document.getElementById("screen");
  const currentResult = eval(screenDiv.innerHTML);
  screenDiv.innerHTML = currentResult * -1;
}

function percentButtonPressed() {
  const screenDiv = document.getElementById("screen");
  const currentResult = eval(screenDiv.innerHTML);
  screenDiv.innerHTML = currentResult / 100;
}

document.addEventListener("keydown", (event) => {
  if (event.key === "Enter") {
    equalButtonPressed();
  }
  if (event.key === "Backspace") {
    clearButtonPressed();
  }
  const operatorSymbols = ["/", "*", "+", "-", "."];
  if (!isNaN(event.key) || operatorSymbols.includes(event.key)) {
    numButtonPressed(event.key);
  }
});