implement json.Marshaler for data.Undefined and data.Null

data/value.go

@@ -53,6 +53,11 @@ func (v Map) Key(k string) Value {
 	return result
 }
 
+// Marshal ---------
+
+func (v Undefined) MarshalJSON() ([]byte, error) { return []byte("null"), nil }
+func (v Null) MarshalJSON() ([]byte, error)      { return []byte("null"), nil }
+
 // Truthy ----------
 
 func (v Undefined) Truthy() bool { return false }

data/value_test.go

@@ -1,6 +1,7 @@
 package data
 
 import (
+	"encoding/json"
 	"reflect"
 	"testing"
 )
@@ -17,6 +18,13 @@ var (
 	_ Value = Map{}
 )
 
+// Ensure custom marshalers are implemented
+
+var (
+	_ json.Marshaler = Undefined{}
+	_ json.Marshaler = Null{}
+)
+
 func TestKey(t *testing.T) {
 	tests := []struct {
 		input    interface{}
@@ -52,3 +60,25 @@ func TestIndex(t *testing.T) {
 		}
 	}
 }
+
+func TestCustomMarhshaling(t *testing.T) {
+	tests := []struct {
+		input    interface{}
+		expected interface{}
+	}{
+		{Null{}, []byte("null")},
+		{Undefined{}, []byte("null")},
+	}
+
+	for _, test := range tests {
+		actual, err := json.Marshal(test.input)
+		if err != nil {
+			t.Errorf("unexpected error: %#v", err.Error())
+		}
+
+		if !reflect.DeepEqual(test.expected, actual) {
+			t.Errorf("%v => %#v, expected %#v", test.input, actual, test.expected)
+		}
+	}
+	}
+}