MongoDB aggregate query with a where in node.js

I have the following mongodb query in node.js which gives me a list of unique zip codes with a count of how many times the zip code appears in the database.

collection.aggregate( [
    {
        $group: {
            _id: "$Location.Zip",
            count: { $sum: 1 }
        }
    },
    { $sort: { _id: 1 } },
    { $match: { count: { $gt: 1 } } }
], function ( lookupErr, lookupData ) {
        if (lookupErr) {
            res.send(lookupErr);
            return;
        }
        res.send(lookupData.sort());
    });
});

How can this query be modified to return one specific zip code? I've tried the condition clause but have not been able to get it to work.

Aggregations that require filtered results can be done with the $match operator. Without tweaking what you already have, I would suggest just sticking in a $match for the zip code you want returned at the top of the aggregation list.

collection.aggregate( [
{   
    $match: {
        zip: 47421
    }
},
{
    $group: {
...

This example will result in every aggregation operation after the $match working on only the data set that is returned by the $match of the zip key to the value 47421.