Added @keys and @values modifiers

The "@keys" and "@values" modifiers converts an object into an
array of its keys or values respectively.

Take this json for example:

{"first":"Tom","last":"Smith"}

@keys   -> ["first","last"]
@values -> ["Tom","Smith"]

This feature was requested in #161.

README.md

@@ -200,6 +200,8 @@ There are currently the following built-in modifiers:
 - `@valid`: Ensure the json document is valid.
 - `@flatten`: Flattens an array.
 - `@join`: Joins multiple objects into a single object.
+- `@keys`: Returns an array of keys for an object.
+- `@values`: Returns an array of values for an object.
 
 ### Modifier arguments
 

SYNTAX.md

@@ -236,6 +236,8 @@ There are currently the following built-in modifiers:
 - `@valid`: Ensure the json document is valid.
 - `@flatten`: Flattens an array.
 - `@join`: Joins multiple objects into a single object.
+- `@keys`: Returns an array of keys for an object.
+- `@values`: Returns an array of values for an object.
 
 #### Modifier arguments
 

gjson.go

@@ -2565,6 +2565,8 @@ var modifiers = map[string]func(json, arg string) string{
 	"flatten": modFlatten,
 	"join":    modJoin,
 	"valid":   modValid,
+	"keys":    modKeys,
+	"values":  modValues,
 }
 
 // AddModifier binds a custom modifier command to the GJSON syntax.
@@ -2721,6 +2723,58 @@ func modFlatten(json, arg string) string {
 	return bytesString(out)
 }
 
+// @keys extracts the keys from an object.
+//  {"first":"Tom","last":"Smith"} -> ["first","last"]
+func modKeys(json, arg string) string {
+	v := Parse(json)
+	if !v.Exists() {
+		return "[]"
+	}
+	obj := v.IsObject()
+	var out strings.Builder
+	out.WriteByte('[')
+	var i int
+	v.ForEach(func(key, _ Result) bool {
+		if i > 0 {
+			out.WriteByte(',')
+		}
+		if obj {
+			out.WriteString(key.Raw)
+		} else {
+			out.WriteString("null")
+		}
+		i++
+		return true
+	})
+	out.WriteByte(']')
+	return out.String()
+}
+
+// @values extracts the values from an object.
+//   {"first":"Tom","last":"Smith"} -> ["Tom","Smith"]
+func modValues(json, arg string) string {
+	v := Parse(json)
+	if !v.Exists() {
+		return "[]"
+	}
+	if v.IsArray() {
+		return json
+	}
+	var out strings.Builder
+	out.WriteByte('[')
+	var i int
+	v.ForEach(func(_, value Result) bool {
+		if i > 0 {
+			out.WriteByte(',')
+		}
+		out.WriteString(value.Raw)
+		i++
+		return true
+	})
+	out.WriteByte(']')
+	return out.String()
+}
+
 // @join multiple objects into a single object.
 //   [{"first":"Tom"},{"last":"Smith"}] -> {"first","Tom","last":"Smith"}
 // The arg can be "true" to specify that duplicate keys should be preserved.

gjson_test.go

@@ -2207,3 +2207,31 @@ func TestIssue240(t *testing.T) {
 	parsed = Parse(arrayData)
 	assert(t, len(parsed.Get("params.data").Array()) == 1)
 }
+
+func TestKeysValuesModifier(t *testing.T) {
+	var json = `{
+		"1300014": {
+		  "code": "1300014",
+		  "price": 59.18,
+		  "symbol": "300014",
+		  "update": "2020/04/15 15:59:54",
+		},
+		"1300015": {
+		  "code": "1300015",
+		  "price": 43.31,
+		  "symbol": "300015",
+		  "update": "2020/04/15 15:59:54",
+		}
+	  }`
+	assert(t, Get(json, `@keys`).String() == `["1300014","1300015"]`)
+	assert(t, Get(``, `@keys`).String() == `[]`)
+	assert(t, Get(`"hello"`, `@keys`).String() == `[null]`)
+	assert(t, Get(`[]`, `@keys`).String() == `[]`)
+	assert(t, Get(`[1,2,3]`, `@keys`).String() == `[null,null,null]`)
+
+	assert(t, Get(json, `@values.#.code`).String() == `["1300014","1300015"]`)
+	assert(t, Get(``, `@values`).String() == `[]`)
+	assert(t, Get(`"hello"`, `@values`).String() == `["hello"]`)
+	assert(t, Get(`[]`, `@values`).String() == `[]`)
+	assert(t, Get(`[1,2,3]`, `@values`).String() == `[1,2,3]`)
+}