how to assume Null fields

[nestorvaldez]

Im working with a MySql Database with PHP, but, right now I need to make some calculations in the client area, I mean the system must give the results before to send to the database, I've got it using Javascript. This is just the explanation what Im doing...
My problem is that I have some decimal(3.2) fields in MySql with NULL. I want Java script in the calculation take the null value not like a 0. This is because I'm making some avg calculations, and I have two Fields to make the avg calculation, if the fields are >=0 it will promediate considering this field if some of the fields is not 0 (NULL.  it take the value from DB) it will not considerate it to promediate.

EX1. REG_PRA1 =80
    REG_PRA2 =90
    In this case it must calc (80+90) /2 = 85

EX2. REG_PRA1 =80
    REG_PRA2 =0
    In this case it must calc (80+0) /2 = 40

EX3. REG_PRA1 =80
    REG_PRA2 =NULL
    In this case it must calc (80+90) /1 = 80

Im using this code: the code is working well, the problem is takin the value (0 or null) from the DB) it take both (0 and null) like 0.0
I need to solve it to make the correct average.

var reg_pra1 = document.Actualizar.reg_pra1.value;
var reg_pra2 = document.Actualizar.reg_pra2.value;

var count=0;
var sum=0;

if(reg_pra1>=0){
count=count+1;
sum = reg_pra1 * 1;
}

if(reg_pra2>=0){
count=count+1;
sum = (sum * 1) + (reg_pra2 * 1);
}

if(sum>0){
var prompra = sum/count;
prompra.toFixed(1)
document.Actualizar.reg_promp.value = prompra.toFixed(1);
}

[fenway]

The question is how you get back NULL vs zero from your DB to begin with.

[nestorvaldez]

so, how is it?

[fenway]

I'm confused... .value in JS for an INPUT will never be NULL.