Added NDJSON decoder (#1281)

pkg/yqlib/doc/usage/convert.md

@@ -1,6 +1,8 @@
 # JSON
 
-Encode and decode to and from JSON. Note that YAML is a _superset_ of JSON - so `yq` can read any json file without doing anything special.
+Encode and decode to and from JSON. Note that, unless you have multiple JSON documents in a single file, YAML is a _superset_ of JSON - so `yq` can read any json file without doing anything special.
+
+If you do have mulitple JSON documents in a single file (e.g. NDJSON) then you will need to explicity use the json parser `-p=json`.
 
 This means you don't need to 'convert' a JSON file to YAML - however if you want idiomatic YAML styling, then you can use the `-P/--prettyPrint` flag, see examples below.
 
@@ -130,3 +132,129 @@ will output
 {"whatever":"cat"}
 ```
 
+## Roundtrip NDJSON
+Unfortunately the json encoder strips leading spaces of values.
+
+Given a sample.json file of:
+```json
+{"this": "is a multidoc json file"}
+{"each": ["line is a valid json document"]}
+{"a number": 4}
+
+```
+then
+```bash
+yq -p=json -o=json -I=0 sample.json
+```
+will output
+```yaml
+{"this":"is a multidoc json file"}
+{"each":["line is a valid json document"]}
+{"a number":4}
+```
+
+## Roundtrip multi-document JSON
+The NDJSON parser can also handle multiple multi-line json documents in a single file!
+
+Given a sample.json file of:
+```json
+{
+	"this": "is a multidoc json file"
+}
+{
+	"it": [
+		"has",
+		"consecutive",
+		"json documents"
+	]
+}
+{
+	"a number": 4
+}
+
+```
+then
+```bash
+yq -p=json -o=json -I=2 sample.json
+```
+will output
+```yaml
+{
+  "this": "is a multidoc json file"
+}
+{
+  "it": [
+    "has",
+    "consecutive",
+    "json documents"
+  ]
+}
+{
+  "a number": 4
+}
+```
+
+## Update a specific document in a multi-document json
+Documents are indexed by the `documentIndex` or `di` operator.
+
+Given a sample.json file of:
+```json
+{"this": "is a multidoc json file"}
+{"each": ["line is a valid json document"]}
+{"a number": 4}
+
+```
+then
+```bash
+yq -p=json -o=json -I=0 '(select(di == 1) | .each ) += "cool"' sample.json
+```
+will output
+```yaml
+{"this":"is a multidoc json file"}
+{"each":["line is a valid json document","cool"]}
+{"a number":4}
+```
+
+## Find and update a specific document in a multi-document json
+Use expressions as you normally would.
+
+Given a sample.json file of:
+```json
+{"this": "is a multidoc json file"}
+{"each": ["line is a valid json document"]}
+{"a number": 4}
+
+```
+then
+```bash
+yq -p=json -o=json -I=0 '(select(has("each")) | .each ) += "cool"' sample.json
+```
+will output
+```yaml
+{"this":"is a multidoc json file"}
+{"each":["line is a valid json document","cool"]}
+{"a number":4}
+```
+
+## Decode NDJSON
+Given a sample.json file of:
+```json
+{"this": "is a multidoc json file"}
+{"each": ["line is a valid json document"]}
+{"a number": 4}
+
+```
+then
+```bash
+yq -p=json sample.json
+```
+will output
+```yaml
+this: is a multidoc json file
+---
+each:
+    - line is a valid json document
+---
+a number: 4
+```
+

pkg/yqlib/doc/usage/headers/convert.md

@@ -1,5 +1,7 @@
 # JSON
 
-Encode and decode to and from JSON. Note that YAML is a _superset_ of JSON - so `yq` can read any json file without doing anything special.
+Encode and decode to and from JSON. Note that, unless you have multiple JSON documents in a single file, YAML is a _superset_ of JSON - so `yq` can read any json file without doing anything special.
+
+If you do have mulitple JSON documents in a single file (e.g. NDJSON) then you will need to explicity use the json parser `-p=json`.
 
 This means you don't need to 'convert' a JSON file to YAML - however if you want idiomatic YAML styling, then you can use the `-P/--prettyPrint` flag, see examples below.