Uncaught TypeError: Cannot read property ‘value’ of null

I am unsure which of them is wrong because you did not provide your HTML, but one of these does not exist:

var str = document.getElementById("cal_preview").value;
var str1 = document.getElementById("year").value;
var str2 = document.getElementById("holiday").value;
var str3 = document.getElementById("cal_option").value;

There is either no element with the id cal_preview, year, holiday, cal_option, or some combination.

Therefore, JavaScript is unable to read the value of something that does not exist.

EDIT:

If you want to check that the element exists first, you could use an if statement for each:

var str,
element = document.getElementById('cal_preview');
if (element != null) {
    str = element.value;
}
else {
    str = null;
}

You could obviously change the else statement if you want or have no else statement at all, but that is all about preference.