Add Reduce Object To Just Entries Of A Specific Type as a jq TIL

README.md

@@ -10,7 +10,7 @@ pairing with smart people at Hashrocket.
 
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-_1272 TILs and counting..._
+_1273 TILs and counting..._
 
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@@ -478,6 +478,7 @@ _1272 TILs and counting..._
 ### jq
 
 - [Extract A List Of Values](jq/extract-a-list-of-values.md)
+- [Reduce Object To Just Entries Of A Specific Type](jq/reduce-object-to-just-entries-of-a-specific-type.md)
 
 ### Kitty
 

jq/reduce-object-to-just-entries-of-a-specific-type.md

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+# Reduce Object To Just Entries Of A Specific Type
+
+Let's say I have a large JSON data file with a ton of top-level fields of
+varying types. It can be hard to wade through it as is. [The `jq`
+utility](https://stedolan.github.io/jq/manual/) can help. I can filter a JSON
+object down to just the fields of a certain type.
+
+For instance, I may want to start with a view of the JSON that is restricted to
+just the values of type `array`.
+
+To do this, I need to use a couple different `jq` helper functions.
+
+```bash
+jq '. | to_entries | map(select(.value | type | match("array"))) | from_entries' data.json
+```
+
+This starts with `to_entries` to convert an object into an array of key-value
+pairs. I then `map` over those pairs selecting just the pairs whose value
+matches the `"array"` type. I then use `from_entries` to turn the reduced array
+back into an object.
+
+There is a less verbose way to do the above. The `to_entries` and
+`from_entries` can be collapsed into a `with_entries` that wraps the `map`
+call.
+
+```bash
+jq '. | with_entries(map(select(.value | type | match("array")))' data.json
+```