Your function's logic doesn't make sense as it is, but maybe because it's a sample. I'll first guess what you are trying to achieve, then show you a possible solution.

First, regarding the name of your function validateFields, I suppose you need to check if entries (ns1 and ns2 in your example) contain valid data.

So, you might want your function to return a boolean (true or false) if the fields are erroneous.

Your current function doesn't check the 2 entries, and there is only a return when nameserverone is null (which doesn't make sense, or maybe it's what you want).

"use strict";

function validateFields() {
    var nameserverone = document.getElementById("ns1").value;
    var nameservertwo = document.getElementById("ns2").value;

    if(nameserverone === null || nameserverthow === null) {
           // one of the entry is empty, we return false
           console.log("one of the servers' name is null");
           return false;
       }
       else {
           // both entries contain data, should be Ok
           return true;
       }       
}

I would recommend to try having only ONE return in your functions no matter what, such fix would do:

"use strict";

function validateFields() {
    var nameserverone = document.getElementById("ns1").value;
    var nameservertwo = document.getElementById("ns2").value;

    var status;

    if(nameserverone === null || nameservertwo === null) {
           // one of the entry is empty, we return false
           console.log("one of the servers' name is null");
           status = false;
    }
    else {
           // both entries contain data, should be Ok
           status = true;
    }       

    return status;
}