Add Find All Objects In An Array Where Key Is Set as a jq TIL

README.md

@@ -10,7 +10,7 @@ pairing with smart people at Hashrocket.
 
 For a steady stream of TILs, [sign up for my newsletter](https://crafty-builder-6996.ck.page/e169c61186).
 
-_1290 TILs and counting..._
+_1291 TILs and counting..._
 
 ---
 
@@ -488,6 +488,7 @@ _1290 TILs and counting..._
 - [Count Each Collection In A JSON Object](jq/count-each-collection-in-a-json-object.md)
 - [Count The Number Of Things In A JSON File](jq/count-the-number-of-things-in-a-json-file.md)
 - [Extract A List Of Values](jq/extract-a-list-of-values.md)
+- [Find All Objects In An Array Where Key Is Set](jq/find-all-objects-in-an-array-where-key-is-set.md)
 - [Find All Objects With A Matching Key Value Pair](jq/find-all-objects-with-a-matching-key-value-pair.md)
 - [Reduce Object To Just Entries Of A Specific Type](jq/reduce-object-to-just-entries-of-a-specific-type.md)
 

jq/find-all-objects-in-an-array-where-key-is-set.md

@@ -0,0 +1,50 @@
+# Find All Objects In An Array Where Key Is Set
+
+Let's say we have a large array of objects. And the data in those objects,
+while generally having the same shape, does not always have certain values set.
+
+For instance, here is some data where the `token` is sometimes set, sometimes
+`null`, and sometimes missing altogether.
+
+```json
+[
+  {
+    "id": 1,
+    "token": "abc"
+  },
+  {
+    "id": 3,
+    "token": null
+  },
+  {
+    "id": 5
+  },
+  ...
+]
+```
+
+We can find out how many objects in this collection have the `token` key set to
+an actual value with [a `jq` query](https://stedolan.github.io/jq/manual) like
+the following.
+
+```bash
+jq '. | map(select(has("token") and .token != null)) | length' data.json
+```
+
+This maps over each object selecting those where it has `token` and `token` is
+not `null`.
+
+We can instead produce the inverse count—those objects where `token` is not set
+to a value—with the `not` operator.
+
+```bash
+jq '. | map(select(has("token") and .token != null | not)) | length' data.json
+```
+
+If you want to inspect the array of objects that either of these queries filters down to, you can drop the `| length` part.
+
+```bash
+jq '. | map(select(has("token") and .token != null))' data.json
+```
+
+Here is [a live example](https://jqterm.com/?query=.%20%7C%20map%28select%28has%28%22token%22%29%20and%20.token%20!%3D%20null%20%7C%20not%29%29).