`Couchbase` comes with a big DB of beers. The 2ms fetch time are impressive but I don't understand what the following map function actually does:

```
function(doc, meta) {
  switch(doc.type) {
  case "brewery":
    emit([meta.id]);
    break;
  case "beer":
    if (doc.brewery_id) {
      emit([doc.brewery_id, meta.id]);
    }
    break;
  }
}

```

I can't wrap my brain around it. Where is this `meta` thing coming from?

![enter image description here](https://i.stack.imgur.com/vmvcN.png)