Javascript verification when variable called may be blank

Question

I have a numeric grid question (QA7) that also has a Not Applicable option (QA7DK)  from the Question Library.  In the next numeric grid question (QA8) I am returning an error message if the QA8 total doesn't match the QA7 total.  That seems to be working if a response was given at QA7.  However, if QA7DK was selected there is no error at QA8 but I just can't move forward.  I'm sure it has to do with the A7 variables being blank but not sure how to fix it.

2nd question, how can I also return the QA7TOT in the error message?

var QA7TOT=[% QA7_r1_c1 %]+[% QA7_r2_c1 %]+[% QA7_r3_c1 %];
var QA8Row1=SSI_GetValue("QA8_r1_c1");
var QA8Row2=SSI_GetValue("QA8_r2_c1");
var QA8TOT=QA8Row1+QA8Row2;
if ([% QA7DK_1 %]==0 && QA8TOT!=QA7TOT)
{strErrorMessage="The Total must match the total of the previous question";}

Comments

One easy way to handle blank Sawtooth Script responses in JavaScript is to surround them with the Number function.  Because Number returns 0 when no arguments are provided, this will effectively convert blanks to zeroes:

Number([% NumericField %])

Is this enough for the error message?

var QA7TOT = ...;
strErrorMessage = "The total must be " + QA7TOT + ".";

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Perfect!  I knew converting it to a number was the answer I just couldn't remember how to apply it.  Error message is exactly what I needed (again, just having a brain freeze).  Thanks as always!