You've used theResult === 0 where you should have used theResult = 0.
The === is for comparisons. When you want to change the value of a variable, you use the assignment operator =.
Like @zzzzBov said, the simplest thing to do is replace the whole thing with
theResult = isFinite(theResult) && theResult || 0; // updated
That glosses over the subtleties of NaN however; if the user is finished and the result really is NaN, showing that as 0 is not really correct.
