JavaScript returns NaN even if input is an integer

Answer 1

document.getElementById("sidea") doesn't returns a number, but an element. Use document.getElementById("sidea").value instead to get the element value.

So:

function goldenFunctiona() {
    var sideaVal = document.getElementById("sidea").value;

    var a = parseInt(sideaVal , 10); // Get parseInt's return value or 'a' will still be a string
    var b = ((a + a) * (Math.pow(5, 0.5))) / 2;

    document.getElementById("b").innerHTML = b;
}