Add tostr and fromstr modifiers

For wrapping and unwrapping json strings

README.md

@@ -204,6 +204,8 @@ There are currently the following built-in modifiers:
 - `@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.
+- `@tostr`: Converts json to a string. Wraps a json string.
+- `@fromstr`: Converts a string from json. Unwraps a json string.
 
 ### Modifier arguments
 

SYNTAX.md

@@ -238,6 +238,8 @@ There are currently the following built-in modifiers:
 - `@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.
+- `@tostr`: Converts json to a string. Wraps a json string.
+- `@fromstr`: Converts a string from json. Unwraps a json string.
 
 #### Modifier arguments
 

gjson.go

@@ -2669,6 +2669,8 @@ var modifiers = map[string]func(json, arg string) string{
 	"valid":   modValid,
 	"keys":    modKeys,
 	"values":  modValues,
+	"tostr":   modToStr,
+	"fromstr": modFromStr,
 }
 
 // AddModifier binds a custom modifier command to the GJSON syntax.
@@ -2954,6 +2956,22 @@ func modValid(json, arg string) string {
 	return json
 }
 
+// @fromstr converts a string to json
+//   "{\"id\":1023,\"name\":\"alert\"}" -> {"id":1023,"name":"alert"}
+func modFromStr(json, arg string) string {
+	if !Valid(json) {
+		return ""
+	}
+	return Parse(json).String()
+}
+
+// @tostr converts a string to json
+//   {"id":1023,"name":"alert"} -> "{\"id\":1023,\"name\":\"alert\"}"
+func modToStr(str, arg string) string {
+	data, _ := json.Marshal(str)
+	return string(data)
+}
+
 // stringHeader instead of reflect.StringHeader
 type stringHeader struct {
 	data unsafe.Pointer

gjson_test.go

@@ -2461,3 +2461,9 @@ func TestArrayKeys(t *testing.T) {
 	})
 	assert(t, i == N)
 }
+
+func TestToFromStr(t *testing.T) {
+	json := `{"Message":"{\"Records\":[{\"eventVersion\":\"2.1\"}]"}`
+	res := Get(json, "Message.@fromstr.Records.#.eventVersion.@tostr").Raw
+	assert(t, res == `["\"2.1\""]`)
+}