what is error of this? It give me wrong results
<html>
<head>
<title>Calculator</title>
<script language='JavaScript'>
var number='0';
var start=0;
var firstNumber=0;
var result=0;
var sign='';
calculator.results.value='0'
function show(number,sign){
if (sign !=''){
calculator.results.value='';
calculator.results.value=number;
}
else{
calculator.results.value=calculator.results.value+number;
}
//start++
}
function add(){
sign='+';
firstNumber=parseFloat(calculator.results.value);
calculator.results.value='';
}
function subtract(){
sign='-';
firstNumber=parseFloat(calculator.results.value);
calculator.results.value='';
}
function equal(sign){
if (sign='+'){
result=parseFloat(calculator.results.value)+firstNumber;
calculator.results.value=result;
}
if (sign='-'){
result=firstNumber-parseFloat(calculator.results.value);
calculator.results.value=result;
}
}
</script>
</head>
<body>
<form name="calculator">
Calculator
<br/>
<input type="text" name="results" value="" />
<br/>
<br/>
<input type="button" value="1" onclick="show('1','')" />
<input type="button" value="2" onclick="show('2','')" />
<input type="button" value="3" onclick="show('3','')" />
<br/>
<input type="button" value="4" onclick="show('4','')" />
<input type="button" value="5" onclick="show('5','')" />
<input type="button" value="6" onclick="show('6','')" />
<br/>
<input type="button" value="7" onclick="show('7','')" />
<input type="button" value="8" onclick="show('8','')" />
<input type="button" value="9" onclick="show('9','')" />
<br/>
<input type="button" value="+" onclick="add()" />
<input type="button" value="-" onclick="subtract()" />
<input type="button" value="=" onclick="equal(sign)" />
<br/>
</form>
</body>
</html>
