Alright, first things first, we need to bring the Firebase library into our project. Once that's done, we can access the Firestore library with this little snippet:

const firebase = require('firebase');
require('firebase/firestore');

Next up, we need to initialize Firebase to use Firestore. Firebase usually gives us all the config parameters we need. Here's a general idea of what the initialization code looks like:

const firebaseConfig = {
    apiKey: "API_KEY",
    authDomain: "PROJECT_ID.firebaseapp.com",
    projectId: "PROJECT_ID",
};

firebase.initializeApp(firebaseConfig);

let db = firebase.firestore();

Don't forget to swap out "API_KEY" and "PROJECT_ID" with your actual app's details.

Now, let's talk about compound queries. These are super handy for filtering data based on multiple fields. You can create some pretty complex rules to fetch exactly what you need from your database.

Here's an example of what a compound query might look like:

let query = db.collection('coll')
                .where('field1', '==', 'value1')
                .where('field2', '>', 'value2')
                .where('field3', 'array-contains', 'value3');

query.get().then((querySnapshot) => {
    querySnapshot.forEach((doc) => {
        console.log(doc.id, " => ", doc.data());
    });
});

In this snippet, 'coll' is the collection you're querying. The .where('field', 'operation', 'value') part is where the magic happens.

Here, 'field' is the field in your documents you want to query, 'operation' is the comparison operator, and 'value' is what you're comparing against.

The .get().then() function fetches the results and prints each document's id and data.

The 'operation' can be one of the following:

• <, <=, ==, >, >= for comparisons
• array-contains to check if an array field has a certain value
• in to see if a field value is in a specific array
• array-contains-any to check if an array field has any value from a given array

When you get results from a query, you can handle them using Promises. In our example, .get().then((querySnapshot) => {}) fetches the results and then processes them with the function inside then(). You can loop over the Snapshot to access each document's data and metadata.

Keep in mind that compound queries might need extra index configurations in Firestore, especially when you're filtering on multiple fields. If your query needs an index, you'll get an error message with a link to create the index in the Firebase Console. Just follow the link and click "Create" to add the index.