script.js

const expressionDisplay = document.querySelector('#expression');
const resultDisplay = document.querySelector('#result');
const keys = document.querySelectorAll('.key');

let expression = '';
let justCalculated = false;

function formatResult(value) {
  if (!Number.isFinite(value)) return 'Error';
  return Number.isInteger(value) ? String(value) : String(Number(value.toFixed(10)));
}

function evaluate(currentExpression) {
  const normalized = currentExpression.replace(/%/g, '/100').replace(/\^/g, '**');
  if (!normalized || !/^[0-9+\-*/%.()\s]+$/.test(normalized.replace(/\*\*/g, ''))) return null;
  try {
    const value = Function(`"use strict"; return (${normalized})`)();
    return typeof value === 'number' && Number.isFinite(value) ? value : null;
  } catch {
    return null;
  }
}

function updateDisplay() {
  expressionDisplay.textContent = expression || '0';
  const preview = evaluate(expression);
  const looksComplete = /[0-9)%]$/.test(expression);
  resultDisplay.textContent = preview === null ? (expression ? (looksComplete ? 'Error' : '...') : '0') : formatResult(preview);
}

function appendValue(value) {
  if (justCalculated && /[0-9.]/.test(value)) expression = '';
  justCalculated = false;

  const lastCharacter = expression.at(-1);
  if (/[+\-*/^]/.test(value) && /[+\-*/^]/.test(lastCharacter)) {
    expression = expression.slice(0, -1);
  }
  if (value === '.' && (lastCharacter === '.' || /\.[0-9]*$/.test(expression.split(/[+\-*/^]/).at(-1)))) return;
  expression += value;
  updateDisplay();
}

function clearCalculator() {
  expression = '';
  justCalculated = false;
  updateDisplay();
}

function deleteLast() {
  expression = expression.slice(0, -1);
  justCalculated = false;
  updateDisplay();
}

function squareRoot() {
  const value = evaluate(expression);
  if (value === null || value < 0) {
    resultDisplay.textContent = 'Error';
    return;
  }
  expression = formatResult(Math.sqrt(value));
  justCalculated = true;
  updateDisplay();
}

function calculate() {
  const value = evaluate(expression);
  if (value === null) {
    resultDisplay.textContent = expression ? 'Error' : '0';
    return;
  }
  expression = formatResult(value);
  justCalculated = true;
  updateDisplay();
}

function handleInput(value, action) {
  if (action === 'clear') clearCalculator();
  else if (action === 'delete') deleteLast();
  else if (action === 'equals') calculate();
  else if (action === 'sqrt') squareRoot();
  else appendValue(value);
}

keys.forEach((key) => {
  key.addEventListener('click', () => handleInput(key.dataset.value, key.dataset.action));
});

document.addEventListener('keydown', (event) => {
  const keyMap = { Enter: 'equals', '=': 'equals', Escape: 'clear', Backspace: 'delete', Delete: 'clear' };
  const action = keyMap[event.key];
  const value = /[0-9+\-*/%^().]/.test(event.key) ? event.key : undefined;
  if (!action && value === undefined) return;
  event.preventDefault();
  handleInput(value, action);
});

updateDisplay();