diff --git a/objects/builtin_convert.go b/objects/builtin_convert.go
index 922efc0d..460a9213 100644
--- a/objects/builtin_convert.go
+++ b/objects/builtin_convert.go
@@ -1,7 +1,8 @@
 package objects
 
 func builtinString(args ...Object) (Object, error) {
-	if len(args) != 1 {
+	argsLen := len(args)
+	if !(argsLen == 1 || argsLen == 2) {
 		return nil, ErrWrongNumArguments
 	}
 
@@ -14,11 +15,18 @@ func builtinString(args ...Object) (Object, error) {
 		return &String{Value: v}, nil
 	}
 
+	if argsLen == 2 {
+		if _, ok := args[1].(*String); ok {
+			return args[1], nil
+		}
+	}
+
 	return UndefinedValue, nil
 }
 
 func builtinInt(args ...Object) (Object, error) {
-	if len(args) != 1 {
+	argsLen := len(args)
+	if !(argsLen == 1 || argsLen == 2) {
 		return nil, ErrWrongNumArguments
 	}
 
@@ -31,11 +39,18 @@ func builtinInt(args ...Object) (Object, error) {
 		return &Int{Value: v}, nil
 	}
 
+	if argsLen == 2 {
+		if _, ok := args[1].(*Int); ok {
+			return args[1], nil
+		}
+	}
+
 	return UndefinedValue, nil
 }
 
 func builtinFloat(args ...Object) (Object, error) {
-	if len(args) != 1 {
+	argsLen := len(args)
+	if !(argsLen == 1 || argsLen == 2) {
 		return nil, ErrWrongNumArguments
 	}
 
@@ -48,6 +63,15 @@ func builtinFloat(args ...Object) (Object, error) {
 		return &Float{Value: v}, nil
 	}
 
+	if argsLen == 2 {
+		if _, ok := args[1].(*Float); ok {
+			return args[1], nil
+		} else if _, ok := args[1].(*Int); ok {
+			v, _ := ToFloat64(args[1])
+			return &Float{Value: v}, nil
+		}
+	}
+
 	return UndefinedValue, nil
 }
 
@@ -69,7 +93,8 @@ func builtinBool(args ...Object) (Object, error) {
 }
 
 func builtinChar(args ...Object) (Object, error) {
-	if len(args) != 1 {
+	argsLen := len(args)
+	if !(argsLen == 1 || argsLen == 2) {
 		return nil, ErrWrongNumArguments
 	}
 
@@ -82,11 +107,18 @@ func builtinChar(args ...Object) (Object, error) {
 		return &Char{Value: v}, nil
 	}
 
+	if argsLen == 2 {
+		if _, ok := args[1].(*Char); ok {
+			return args[1], nil
+		}
+	}
+
 	return UndefinedValue, nil
 }
 
 func builtinBytes(args ...Object) (Object, error) {
-	if len(args) != 1 {
+	argsLen := len(args)
+	if !(argsLen == 1 || argsLen == 2) {
 		return nil, ErrWrongNumArguments
 	}
 
@@ -100,5 +132,17 @@ func builtinBytes(args ...Object) (Object, error) {
 		return &Bytes{Value: v}, nil
 	}
 
+	if argsLen == 2 {
+		// bytes(N) => create a new bytes with given size N
+		if n, ok := args[1].(*Int); ok {
+			return &Bytes{Value: make([]byte, int(n.Value))}, nil
+		}
+	
+		v, ok = ToByteSlice(args[1])
+		if ok {
+			return &Bytes{Value: v}, nil
+		}
+	}
+
 	return UndefinedValue, nil
 }
diff --git a/runtime/vm_builtin_test.go b/runtime/vm_builtin_test.go
index 71746bb9..705834d4 100644
--- a/runtime/vm_builtin_test.go
+++ b/runtime/vm_builtin_test.go
@@ -27,6 +27,10 @@ func TestBuiltinFunction(t *testing.T) {
 	expect(t, `out = int([1])`, undefined())
 	expect(t, `out = int({a: 1})`, undefined())
 	expect(t, `out = int(undefined)`, undefined())
+	expect(t, `out = int("-522", 1)`, -522)
+	expect(t, `out = int(undefined, 1)`, 1)
+	expect(t, `out = int(undefined, 1.8)`, undefined())
+	expect(t, `out = int(undefined, undefined)`, undefined())
 
 	expect(t, `out = string(1)`, "1")
 	expect(t, `out = string(1.8)`, "1.8")
@@ -37,6 +41,8 @@ func TestBuiltinFunction(t *testing.T) {
 	expect(t, `out = string([1,8.1,true,3])`, "[1, 8.1, true, 3]")
 	expect(t, `out = string({b: "foo"})`, `{b: "foo"}`)
 	expect(t, `out = string(undefined)`, undefined()) // not "undefined"
+	expect(t, `out = string(1, "-522")`, "1")
+	expect(t, `out = string(undefined, "-522")`, "-522") // not "undefined"
 
 	expect(t, `out = float(1)`, 1.0)
 	expect(t, `out = float(1.8)`, 1.8)
@@ -47,6 +53,11 @@ func TestBuiltinFunction(t *testing.T) {
 	expect(t, `out = float([1,8.1,true,3])`, undefined())
 	expect(t, `out = float({a: 1, b: "foo"})`, undefined())
 	expect(t, `out = float(undefined)`, undefined())
+	expect(t, `out = float("-52.2", 1.8)`, -52.2)
+	expect(t, `out = float(undefined, 1)`, 1.0)
+	expect(t, `out = float(undefined, 1.8)`, 1.8)
+	expect(t, `out = float(undefined, "-52.2")`, undefined())
+	expect(t, `out = float(undefined, undefined)`, undefined())
 
 	expect(t, `out = char(56)`, '8')
 	expect(t, `out = char(1.8)`, undefined())
@@ -57,6 +68,10 @@ func TestBuiltinFunction(t *testing.T) {
 	expect(t, `out = char([1,8.1,true,3])`, undefined())
 	expect(t, `out = char({a: 1, b: "foo"})`, undefined())
 	expect(t, `out = char(undefined)`, undefined())
+	expect(t, `out = char(56, 'a')`, '8')
+	expect(t, `out = char(undefined, '8')`, '8')
+	expect(t, `out = char(undefined, 56)`, undefined())
+	expect(t, `out = char(undefined, "-52.2")`, undefined())
 
 	expect(t, `out = bool(1)`, true)          // non-zero integer: true
 	expect(t, `out = bool(0)`, false)         // zero: true
@@ -83,6 +98,11 @@ func TestBuiltinFunction(t *testing.T) {
 	expect(t, `out = bytes([1])`, undefined())
 	expect(t, `out = bytes({a: 1})`, undefined())
 	expect(t, `out = bytes(undefined)`, undefined())
+	expect(t, `out = bytes("-522", ['8'])`, []byte{'-', '5', '2', '2'})
+	expect(t, `out = bytes(undefined, "-522")`, []byte{'-', '5', '2', '2'})
+	expect(t, `out = bytes(undefined, 1)`, []byte{0})
+	expect(t, `out = bytes(undefined, 1.8)`, undefined())
+	expect(t, `out = bytes(undefined, undefined)`, undefined())
 
 	expect(t, `out = is_error(error(1))`, true)
 	expect(t, `out = is_error(1)`, false)