<div class="calculator">
  <p>wolkanca / Hesap Makinesi</p>
  <div class="calc-row">
    <div class="screen">0123456789</div>
  </div>
  
  <div class="calc-row">
    <div class="button">C</div><div class="button">CE</div><div class="button backspace">geri</div><div class="button plus-minus">+/-</div><div class="button">%</div>
  </div>
  
  <div class="calc-row">
    <div class="button">7</div><div class="button">8</div><div class="button">9</div><div class="button divice">/</div><div class="button root">x<sup>2</sup></div>
  </div>
  
  <div class="calc-row">
    <div class="button">4</div><div class="button">5</div><div class="button">6</div><div class="button multiply">*</div><div class="button inverse">1/x</div>
  </div>
  
  <div class="calc-row">
    <div class="button">1</div><div class="button">2</div><div class="button">3</div><div class="button">-</div><div class="button pi">pi</div>
  </div>
  
  <div class="calc-row">
    <div class="button zero">0</div><div class="button decimal">.</div><div class="button">+</div><div class="button equal">=</div>
  </div>
</div>

body {
  background-color: darkkhaki;
}

p {
  margin: 0;
  font-family: Pacifico;
  position: relative;
  left: 5%;
}

.calculator {
  position: relative;
  margin: 1em auto;
  padding: 1em 0;
  display: block-inline;
  width: 350px;
  background-color: #444;
  border-radius: 25px;
  box-shadow: 5px 5px 15px 3px #111;
  font-family: 'Oxygen';
}

.calc-row {
  text-align: center;
}

.calc-row div.screen {
  font-family: Droid Sans Mono;
  display: table;
  width: 85%;
  background-color: #aaa;
  text-align: right;
  font-size: 2em;
  min-height: 1.2em;
  margin-left: 0.5em;
  padding-right: 0.5em;
  border: 1px solid #888;
  color: #333;
}

.calc-row div {
  text-align: center;
  display: inline-block;
  font-weight: bold;
  border: 1px solid #555;
  background-color: #eee;
  padding: 10px 0;
  margin: 7px 5px;
  border-radius: 15px;
  box-shadow: 2px 2px 1px 1px #222;
  width: 50px;
}
.calc-row .button {
cursor: pointer;
}
.calc-row div.zero {
  width: 112px;
}

.calc-row div.zero {
  margin-right: 5px;
}

$(document).ready(function() {
  var result = 0;
  var prevEntry = 0;
  var operation = null;
  var currentEntry = '0';
  updateScreen(result);
  
  $('.button').on('click', function(evt) {
    var buttonPressed = $(this).html();
    console.log(buttonPressed);
    
    if (buttonPressed === "C") {
      result = 0;
      currentEntry = '0';
    } else if (buttonPressed === "CE") {
      currentEntry = '0';
    } else if (buttonPressed === "back") {
      //currentEntry = currentEntry.substring(0, currentEntry.length-1);
    } else if (buttonPressed === "+/-") {
      currentEntry *= -1;
    } else if (buttonPressed === '.') {
      currentEntry += '.';
    } else if (isNumber(buttonPressed)) {
      if (currentEntry === '0') currentEntry = buttonPressed;
      else currentEntry = currentEntry + buttonPressed;
    } else if (isOperator(buttonPressed)) {
      prevEntry = parseFloat(currentEntry);
      operation = buttonPressed;
      currentEntry = '';
    } else if(buttonPressed === '%') {
      currentEntry = currentEntry / 100;
    } else if (buttonPressed === 'sqrt') {
      currentEntry = Math.sqrt(currentEntry);
    } else if (buttonPressed === '1/x') {
      currentEntry = 1 / currentEntry;
    } else if (buttonPressed === 'pi') {
      currentEntry = Math.PI;
    } else if (buttonPressed === '=') {
      currentEntry = operate(prevEntry, currentEntry, operation);
      operation = null;
    }
    
    updateScreen(currentEntry);
  });
});

updateScreen = function(displayValue) {
  var displayValue = displayValue.toString();
  $('.screen').html(displayValue.substring(0, 10));
};

isNumber = function(value) {
  return !isNaN(value);
}

isOperator = function(value) {
  return value === '/' || value === '*' || value === '+' || value === '-';
};

operate = function(a, b, operation) {
  a = parseFloat(a);
  b = parseFloat(b);
  console.log(a, b, operation);
  if (operation === '+') return a + b;
  if (operation === '-') return a - b;
  if (operation === '*') return a * b;
  if (operation === '/') return a / b;
}