Programmatically build dynamic query with Mongoose

I'm trying to build a search from input received from a form.

router.get('/data', function(req, res) {
    var firstName=req.body.firstName,
    lastName=req.body.lastName,
    companyName=req.body.companyName,
    email=req.body.email;
});

I'd like to build a query up from these values, but if the field has no value, I obviously don't want to include it in the search (searching for "" would change the results)

I've tried a couple different things, like building a string to place in:

mongoose.model('customers').find({QUERY STRING WOULD GO HERE} ,function(err, data) {
    if (err) return console.error(err);
    console.log(data);
});

But that doesn't seem to work properly. I also tried "stacking" search queries like this:

if(firstName !="") {
    mongoose.model('customers').find({firstName: firstName})
}

and then executing the search like this:

mongoose.model('customers').exec(function(err, customer){
    console.log(customer);
});

But that causes 500 errors (and I'm not sure if there's any more info I can get from them).

Please help a Newbie dynamically build a mongoose search query :(