Fix undefined value compatibility

objects/undefined.go

@@ -6,33 +6,33 @@ import "github.com/d5/tengo/compiler/token"
 type Undefined struct{}
 
 // TypeName returns the name of the type.
-func (o Undefined) TypeName() string {
+func (o *Undefined) TypeName() string {
 	return "undefined"
 }
 
-func (o Undefined) String() string {
+func (o *Undefined) String() string {
 	return "<undefined>"
 }
 
 // BinaryOp returns another object that is the result of
 // a given binary operator and a right-hand side object.
-func (o Undefined) BinaryOp(op token.Token, rhs Object) (Object, error) {
+func (o *Undefined) BinaryOp(op token.Token, rhs Object) (Object, error) {
 	return nil, ErrInvalidOperator
 }
 
 // Copy returns a copy of the type.
-func (o Undefined) Copy() Object {
-	return Undefined{}
+func (o *Undefined) Copy() Object {
+	return &Undefined{}
 }
 
 // IsFalsy returns true if the value of the type is falsy.
-func (o Undefined) IsFalsy() bool {
+func (o *Undefined) IsFalsy() bool {
 	return true
 }
 
 // Equals returns true if the value of the type
 // is equal to the value of another object.
-func (o Undefined) Equals(x Object) bool {
+func (o *Undefined) Equals(x Object) bool {
 	_, ok := x.(*Undefined)
 
 	return ok

script/conversion.go

@@ -65,6 +65,8 @@ func objectToInterface(o objects.Object) interface{} {
 
 func interfaceToObject(v interface{}) (objects.Object, error) {
 	switch v := v.(type) {
+	case nil:
+		return undefined, nil
 	case string:
 		return &objects.String{Value: v}, nil
 	case int64:

script/variable.go

@@ -156,7 +156,7 @@ func (v *Variable) Object() objects.Object {
 
 // IsUndefined returns true if the underlying value is undefined.
 func (v *Variable) IsUndefined() bool {
-	_, isUndefined := (*v.value).(objects.Undefined)
+	_, isUndefined := (*v.value).(*objects.Undefined)
 
 	return isUndefined
 }

script/variable_test.go

@@ -19,6 +19,7 @@ type VariableTest struct {
 	BoolValue   bool
 	StringValue string
 	Object      objects.Object
+	IsUndefined bool
 }
 
 func TestVariable(t *testing.T) {
@@ -52,6 +53,14 @@ func TestVariable(t *testing.T) {
 			StringValue: "true",
 			Object:      &objects.Bool{Value: true},
 		},
+		{
+			Name:        "d",
+			Value:       nil,
+			ValueType:   "undefined",
+			StringValue: "<undefined>",
+			Object:      objects.UndefinedValue,
+			IsUndefined: true,
+		},
 	}
 
 	for _, tc := range vars {
@@ -66,6 +75,6 @@ func TestVariable(t *testing.T) {
 		assert.Equal(t, tc.BoolValue, v.Bool())
 		assert.Equal(t, tc.StringValue, v.String())
 		assert.Equal(t, tc.Object, v.Object())
+		assert.Equal(t, tc.IsUndefined, v.IsUndefined())
 	}
-
 }