Javascript Variables in if statement. Numeric versus String

I have a HTML select tag. Depending on the selected item in the listbox I want to write this value in a ordering sheet. The ordering sheet is made of several td: Prod1, Prod2, Prod3 ... It must be simple but I can't see it. How come this code:

var compteur = 0;
function Add_Product() {
   var e = document.getElementById("SelectionProduct");
   var strProduct = e.options[e.selectedIndex].text;
   if (compteur = 0) {
     document.getElementById('Prod1').innerHTML = strProduct;
     compteur++;
   }
}

doesn't execute the line:

document.getElementById('Prod1').innerHTML = strProduct;

But this one, which is of no use for my purpose, does:

var compteur = "0";
function Add_Product() {
   var e = document.getElementById("SelectionProduct");
   var strProduct = e.options[e.selectedIndex].text;
   if (compteur = "0") {
     document.getElementById('Prod1').innerHTML = strProduct;

   }
}

Thanks.