Смотрите строки 32 и 58
<DOCTYPE! html>
<html>
<head>
<meta charset="utf-8">
<title>Calculator</title>
<link rel="stylesheet" href="my.css">
</head>
<body>
<div id="calculator">
<div class="top">
<span class="clear">C</span>
<div class="screen"></div>
</div>
<div class="keys">
<span>7</span>
<span>8</span>
<span>9</span>
<span class="operator">+</span>
<span>4</span>
<span>5</span>
<span>6</span>
<span class="operator">-</span>
<span>1</span>
<span>2</span>
<span>3</span>
<span class="operator">÷</span>
<span>0</span>
<span class="eval">=</span>
<span class="operator">x</span>
</div>
</div>
<div id = "prot"></div>
<script>
var keys = document.querySelectorAll('#calculator span');
var operators = ['+', '-', 'x', '÷'];
var decimalAdded = false;
for(var i = 0; i < keys.length; i++) {
keys[i].onclick = function(e) {
var input = document.querySelector('.screen');
var inputVal = input.innerHTML;
var btnVal = this.innerHTML;
if(btnVal == 'C') {
input.innerHTML = '';
decimalAdded = false;
}
else if(btnVal == '=') {
var equation = inputVal;
var lastChar = equation[equation.length - 1];
equation = equation.replace(/x/g, '*').replace(/÷/g, '/');
if(operators.indexOf(lastChar) > -1 || lastChar == '.')
equation = equation.replace(/.$/, '');
if(equation)
input.innerHTML = eval(equation);
document.querySelector("#prot").innerHTML += equation + "=" + input.innerHTML + "<br>";
decimalAdded = false;
}
else if(operators.indexOf(btnVal) > -1) {
var lastChar = inputVal[inputVal.length - 1];
if(inputVal != '' && operators.indexOf(lastChar) == -1)
input.innerHTML += btnVal;
else if(inputVal == '' && btnVal == '-')
input.innerHTML += btnVal;
if(operators.indexOf(lastChar) > -1 && inputVal.length > 1) {
input.innerHTML = inputVal.replace(/.$/, btnVal);
}
decimalAdded =false;
}
else if(btnVal == '.') {
if(!decimalAdded) {
input.innerHTML += btnVal;
decimalAdded = true;
}
}
else {
input.innerHTML += btnVal;
}
e.preventDefault();
}
}
</script>
</body>
</html>