Add Count Each Collection In A JSON Object as a jq TIL

jq/count-each-collection-in-a-json-object.md

@@ -0,0 +1,34 @@
+# Count Each Collection In A JSON Object
+
+Let's say your JSON file is an object that represents several different
+collections (arrays) of data.
+
+```json
+{
+  "users": [ ... ],
+  "orders": [ ... ],
+  "carts": [ ... ]
+}
+```
+
+We can get a nice summary of the counts of those collections using
+[`jq`](https://stedolan.github.io/jq/). We can do that with the [`with_entries`
+function](https://stedolan.github.io/jq/manual/#to_entries,from_entries,with_entries).
+We preserve the key (name) of each collection and then process each list with
+the `length` function.
+
+```bash
+jq '. | with_entries({ "key": .key, "value": (.value | length)})' data.json
+{
+  "users": 1234,
+  "orders": 5432,
+  "carts": 89
+}
+```
+
+The `with_entries` function essentially maps over each key-value pair
+processing it with the given expression. It will then convert that `{"key":
+some_key, "value": 123}` mapping back into a key-value pair that gets combined
+with all the others.
+
+[source](https://til.simonwillison.net/jq/flatten-nested-json-objects-jq)