Calculator

HTML

let displayValue: string = "0";
let firstOperand: number | null = null;
let operator: string | null = null;
let waitingForSecondOperand: boolean = false;

function inputDigit(digit: string): void {
  if (waitingForSecondOperand) {
    displayValue = digit;
    waitingForSecondOperand = false;
  } else {
    displayValue = displayValue === "0" ? digit : displayValue + digit;
  }
  updateDisplay();
}

function inputOperator(nextOperator: string): void {
  const inputValue: number = parseFloat(displayValue);

  if (operator && waitingForSecondOperand) {
    operator = nextOperator;
    return;
  }

  if (firstOperand === null) {
    firstOperand = inputValue;
  } else if (operator) {
    const result: number = performCalculation(
      firstOperand,
      inputValue,
      operator,
    );
    displayValue = `${parseFloat(result.toFixed(7))}`;
    firstOperand = result;
  }

  operator = nextOperator;
  waitingForSecondOperand = true;
  updateDisplay();
}

function performCalculation(
  firstOperand: number,
  secondOperand: number,
  operator: string,
): number {
  switch (operator) {
    case "+":
      return firstOperand + secondOperand;
    case "-":
      return firstOperand - secondOperand;
    case "*":
      return firstOperand * secondOperand;
    case "/":
      if (secondOperand === 0) {
        alert("Cannot divide by zero");
        return firstOperand;
      }
      return firstOperand / secondOperand;
    case "%":
      return firstOperand % secondOperand;
    default:
      return secondOperand;
  }
}

function calculateResult(): void {
  if (operator === null || waitingForSecondOperand) return;
  const secondOperand: number = parseFloat(displayValue);
  displayValue = `${parseFloat(performCalculation(firstOperand!, secondOperand, operator).toFixed(7))}`;
  operator = null;
  firstOperand = null;
  waitingForSecondOperand = false;
  updateDisplay();
}

function clearDisplay(): void {
  displayValue = "0";
  firstOperand = null;
  operator = null;
  waitingForSecondOperand = false;
  updateDisplay();
}

function invertSign(): void {
  displayValue = (parseFloat(displayValue) * -1).toString();
  updateDisplay();
}

function updateDisplay(): void {
  const display: HTMLElement | null = document.querySelector("#display");
  if (display) {
    display.textContent = displayValue;
  }
}

updateDisplay();

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