calculator.js

let runningTotal = 0;
let buffer = "0";
let previousOperator = null;

const screen = document.querySelector('.screen');

function buttonClick(value) {
  if (isNaN(value)) {
    // this is not a number
    handleSymbol(value);
  } else {
    // this is a number
    handleNumber(value);
  }
  screen.innerText = buffer;
}

function handleSymbol(symbol) {
  switch (symbol) {
    case 'C':
      buffer = '0';
      runningTotal = 0;
      break;
    case '=':
      if (previousOperator === null) {
        // you need two numbers to do math
        return;
      }
      // do the math
      flushOperation(parseInt(buffer));
      previousOperator = null;
      buffer = runningTotal;
      runningTotal = 0;
      break;
    case '←':
      if (buffer.length === 1) {
        buffer = '0';
      } else {
        buffer = buffer.substring(0, buffer.length - 1);
      }
      break;
    case '+':
    case '−':
    case '×':
    case '÷':
      handleMath(symbol);
      break;
  }
}

function handleMath(symbol) {
  if (buffer === '0') {
    // do nothing
    return;
  }

  // turn string into number
  const intBuffer = parseInt(buffer);

  if (runningTotal === 0) {
    runningTotal = intBuffer;
  } else {
    flushOperation(intBuffer);
  }

  previousOperator = symbol;
  buffer = '0';
}

function handleNumber(numberString) {
  if (buffer === '0') {
    buffer = numberString;
  } else {
    buffer += numberString;
  }
}

function flushOperation(intBuffer) {
  if (previousOperator === '+') {
    runningTotal += intBuffer;
  } else if (previousOperator === '−') {
    runningTotal -= intBuffer;
  } else if (previousOperator === '×') {
    runningTotal *= intBuffer;
  } else {
    runningTotal /= intBuffer;
  }
}

function init() {
  document.querySelector('.calc-buttons')
    .addEventListener('click', function (event) {
      buttonClick(event.target.innerText);
    })
}

init();