Before Submit if statement?

On creation of an inventory item, I am trying to do two things, copy a field into two other fields if those two fields are empty/null. Here is what I have:


function beforesubmit(type) {

    var purchasedesc = nlapiGetFieldValue('purchasedescription');

    if (purchasedesc != "null") {
        copy_from = nlapiGetFieldValue('displayname');
        nlapiSetFieldValue('purchasedescription', copy_from);

        var salesdesc = nlapiGetFieldValue('salesdescription');

        if (salesdesc != "null") {
            copy_from = nlapiGetFieldValue('displayname');
            nlapiSetFieldValue('salesdescription', copy_from);

        }
    }
}

The part of this script that is working is copying. That is good enough, but I was hoping to create an if statement leaving the fields alone if they are not empty/null.

The if part of it does not work. From what I have searched around, I think I am close, but the more I look at it the more I think having an if statement in a before submit won't work because there is technically nothing in the field yet.