How to add decimal point from a string into a numerical value

I want to be able to have a decimal point when I enter my numbers through this code and what the code does is that is it just displays numbers when you enter them like if i clicked 5 twice it would be 55 but what if I wanted 5.5, how would I do that?

var number = '';
var number1 = '';
var number2 = '';
function selectNum(num) 
{
    number += num;
    number = parseInt(number);
    document.getElementById("outputbox").innerHTML = number;
}


<button onclick = "selectNum('7')">7</button>
<button onclick = "selectNum('8')">8</button>
<button onclick = "selectNum('9')">9</button>
<button onclick = "selectNum('4')">4</button>
<button onclick = "selectNum('5')">5</button>
<button onclick = "selectNum('6')">6</button>
<button onclick = "selectNum('1')">1</button>
<button onclick = "selectNum('2')">2</button>
<button onclick = "selectNum('3')">3</button>
<button onclick = "selectNum('0')">0</button>
<button onclick = "selectNum('.')">.</button>