How do I pass string value to alert message for each empty field? 

I have w, t, a, c and s. For example, if w is empty, it should give
the alert message "Mandatory field w must be filled out". And same
goes for others.

My javascript:

```
function addvalidation(){
        var w = document.forms["shop"]["w"].value;
        var t = document.forms["shop"]["t"].value;
        var a = document.forms["shop"]["a"].value;
        var c = document.forms["shop"]["c"].value;
        var s = document.forms["shop"]["s"].value;
        var c = document.forms["shop"]["c"].value;

        if (w == "" || t =="" || a == ""|| c == "" || s == "" || c == "") {
                alert("Mandatory fields must be filled out");
                return false;
            }

```