syscall/js: garbage collect references to JavaScript values

The js.Value struct now contains a pointer, so a finalizer can
determine if the value is not referenced by Go any more.

Unfortunately this breaks Go's == operator with js.Value. This change
adds a new Equal method to check for the equality of two Values.
This is a breaking change. The == operator is now disallowed to
not silently break code.

Additionally the helper methods IsUndefined, IsNull and IsNaN got added.

Fixes #35111

Change-Id: I58a50ca18f477bf51a259c668a8ba15bfa76c955
Reviewed-on: https://go-review.googlesource.com/c/go/+/203600
Run-TryBot: Richard Musiol <neelance@gmail.com>
Reviewed-by: Cherry Zhang <cherryyz@google.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

7 files changed, 209 insertions, 64 deletions

diff --git a/src/net/http/roundtrip_js.go b/src/net/http/roundtrip_js.go
index 6331351a83..4dd99651a7 100644
--- a/src/net/http/roundtrip_js.go
+++ b/src/net/http/roundtrip_js.go
@@ -41,7 +41,7 @@ const jsFetchCreds = "js.fetch:credentials"
 // Reference: https://developer.mozilla.org/en-US/docs/Web/API/WindowOrWorkerGlobalScope/fetch#Parameters
 const jsFetchRedirect = "js.fetch:redirect"
 
-var useFakeNetwork = js.Global().Get("fetch") == js.Undefined()
+var useFakeNetwork = js.Global().Get("fetch").IsUndefined()
 
 // RoundTrip implements the RoundTripper interface using the WHATWG Fetch API.
 func (t *Transport) RoundTrip(req *Request) (*Response, error) {
@@ -50,7 +50,7 @@ func (t *Transport) RoundTrip(req *Request) (*Response, error) {
 	}
 
 	ac := js.Global().Get("AbortController")
-	if ac != js.Undefined() {
+	if !ac.IsUndefined() {
 		// Some browsers that support WASM don't necessarily support
 		// the AbortController. See
 		// https://developer.mozilla.org/en-US/docs/Web/API/AbortController#Browser_compatibility.
@@ -74,7 +74,7 @@ func (t *Transport) RoundTrip(req *Request) (*Response, error) {
 		opt.Set("redirect", h)
 		req.Header.Del(jsFetchRedirect)
 	}
-	if ac != js.Undefined() {
+	if !ac.IsUndefined() {
 		opt.Set("signal", ac.Get("signal"))
 	}
 	headers := js.Global().Get("Headers").New()
@@ -132,7 +132,7 @@ func (t *Transport) RoundTrip(req *Request) (*Response, error) {
 		var body io.ReadCloser
 		// The body is undefined when the browser does not support streaming response bodies (Firefox),
 		// and null in certain error cases, i.e. when the request is blocked because of CORS settings.
-		if b != js.Undefined() && b != js.Null() {
+		if !b.IsUndefined() && !b.IsNull() {
 			body = &streamReader{stream: b.Call("getReader")}
 		} else {
 			// Fall back to using ArrayBuffer
@@ -168,7 +168,7 @@ func (t *Transport) RoundTrip(req *Request) (*Response, error) {
 	respPromise.Call("then", success, failure)
 	select {
 	case <-req.Context().Done():
-		if ac != js.Undefined() {
+		if !ac.IsUndefined() {
 			// Abort the Fetch request
 			ac.Call("abort")
 		}
diff --git a/src/syscall/fs_js.go b/src/syscall/fs_js.go
index 91042f10ef..f7079e9d09 100644
--- a/src/syscall/fs_js.go
+++ b/src/syscall/fs_js.go
@@ -259,7 +259,7 @@ func Lchown(path string, uid, gid int) error {
 	if err := checkPath(path); err != nil {
 		return err
 	}
-	if jsFS.Get("lchown") == js.Undefined() {
+	if jsFS.Get("lchown").IsUndefined() {
 		// fs.lchown is unavailable on Linux until Node.js 10.6.0
 		// TODO(neelance): remove when we require at least this Node.js version
 		return ENOSYS
@@ -497,7 +497,7 @@ func fsCall(name string, args ...interface{}) (js.Value, error) {
 		var res callResult
 
 		if len(args) >= 1 { // on Node.js 8, fs.utimes calls the callback without any arguments
-			if jsErr := args[0]; jsErr != js.Null() {
+			if jsErr := args[0]; !jsErr.IsNull() {
 				res.err = mapJSError(jsErr)
 			}
 		}
diff --git a/src/syscall/js/export_test.go b/src/syscall/js/export_test.go
new file mode 100644
index 0000000000..1b5ed3ce84
--- /dev/null
+++ b/src/syscall/js/export_test.go
@@ -0,0 +1,9 @@
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// +build js,wasm
+
+package js
+
+var JSGo = jsGo
diff --git a/src/syscall/js/func.go b/src/syscall/js/func.go
index 6b7f39b878..6c145c9da6 100644
--- a/src/syscall/js/func.go
+++ b/src/syscall/js/func.go
@@ -64,7 +64,7 @@ func init() {
 
 func handleEvent() {
 	cb := jsGo.Get("_pendingEvent")
-	if cb == Null() {
+	if cb.IsNull() {
 		return
 	}
 	jsGo.Set("_pendingEvent", Null())
diff --git a/src/syscall/js/js.go b/src/syscall/js/js.go
index f42a16f0d0..8a04399171 100644
--- a/src/syscall/js/js.go
+++ b/src/syscall/js/js.go
@@ -12,6 +12,7 @@
 package js
 
 import (
+	"runtime"
 	"unsafe"
 )
 
@@ -20,7 +21,7 @@ import (
 // The JavaScript value "undefined" is represented by the value 0.
 // A JavaScript number (64-bit float, except 0 and NaN) is represented by its IEEE 754 binary representation.
 // All other values are represented as an IEEE 754 binary representation of NaN with bits 0-31 used as
-// an ID and bits 32-33 used to differentiate between string, symbol, function and object.
+// an ID and bits 32-34 used to differentiate between string, symbol, function and object.
 type ref uint64
 
 // nanHead are the upper 32 bits of a ref which are set if the value is not encoded as an IEEE 754 number (see above).
@@ -33,21 +34,45 @@ type Wrapper interface {
 }
 
 // Value represents a JavaScript value. The zero value is the JavaScript value "undefined".
+// Values can be checked for equality with the Equal method.
 type Value struct {
-	ref ref
+	_     [0]func() // uncomparable; to make == not compile
+	ref   ref       // identifies a JavaScript value, see ref type
+	gcPtr *ref      // used to trigger the finalizer when the Value is not referenced any more
 }
 
+const (
+	// the type flags need to be in sync with wasm_exec.js
+	typeFlagNone = iota
+	typeFlagObject
+	typeFlagString
+	typeFlagSymbol
+	typeFlagFunction
+)
+
 // JSValue implements Wrapper interface.
 func (v Value) JSValue() Value {
 	return v
 }
 
-func makeValue(v ref) Value {
-	return Value{ref: v}
+func makeValue(r ref) Value {
+	var gcPtr *ref
+	typeFlag := (r >> 32) & 7
+	if (r>>32)&nanHead == nanHead && typeFlag != typeFlagNone {
+		gcPtr = new(ref)
+		*gcPtr = r
+		runtime.SetFinalizer(gcPtr, func(p *ref) {
+			finalizeRef(*p)
+		})
+	}
+
+	return Value{ref: r, gcPtr: gcPtr}
 }
 
-func predefValue(id uint32) Value {
-	return Value{ref: nanHead<<32 | ref(id)}
+func finalizeRef(r ref)
+
+func predefValue(id uint32, typeFlag byte) Value {
+	return Value{ref: (nanHead|ref(typeFlag))<<32 | ref(id)}
 }
 
 func floatValue(f float64) Value {
@@ -73,28 +98,48 @@ func (e Error) Error() string {
 
 var (
 	valueUndefined = Value{ref: 0}
-	valueNaN       = predefValue(0)
-	valueZero      = predefValue(1)
-	valueNull      = predefValue(2)
-	valueTrue      = predefValue(3)
-	valueFalse     = predefValue(4)
-	valueGlobal    = predefValue(5)
-	jsGo           = predefValue(6) // instance of the Go class in JavaScript
+	valueNaN       = predefValue(0, typeFlagNone)
+	valueZero      = predefValue(1, typeFlagNone)
+	valueNull      = predefValue(2, typeFlagNone)
+	valueTrue      = predefValue(3, typeFlagNone)
+	valueFalse     = predefValue(4, typeFlagNone)
+	valueGlobal    = predefValue(5, typeFlagObject)
+	jsGo           = predefValue(6, typeFlagObject) // instance of the Go class in JavaScript
 
 	objectConstructor = valueGlobal.Get("Object")
 	arrayConstructor  = valueGlobal.Get("Array")
 )
 
+// Equal reports whether v and w are equal according to JavaScript's === operator.
+func (v Value) Equal(w Value) bool {
+	return v.ref == w.ref && v.ref != valueNaN.ref
+}
+
 // Undefined returns the JavaScript value "undefined".
 func Undefined() Value {
 	return valueUndefined
 }
 
+// IsUndefined reports whether v is the JavaScript value "undefined".
+func (v Value) IsUndefined() bool {
+	return v.ref == valueUndefined.ref
+}
+
 // Null returns the JavaScript value "null".
 func Null() Value {
 	return valueNull
 }
 
+// IsNull reports whether v is the JavaScript value "null".
+func (v Value) IsNull() bool {
+	return v.ref == valueNull.ref
+}
+
+// IsNaN reports whether v is the JavaScript value "NaN".
+func (v Value) IsNaN() bool {
+	return v.ref == valueNaN.ref
+}
+
 // Global returns the JavaScript global object, usually "window" or "global".
 func Global() Value {
 	return valueGlobal
@@ -232,16 +277,18 @@ func (v Value) Type() Type {
 	if v.isNumber() {
 		return TypeNumber
 	}
-	typeFlag := v.ref >> 32 & 3
+	typeFlag := (v.ref >> 32) & 7
 	switch typeFlag {
-	case 1:
+	case typeFlagObject:
+		return TypeObject
+	case typeFlagString:
 		return TypeString
-	case 2:
+	case typeFlagSymbol:
 		return TypeSymbol
-	case 3:
+	case typeFlagFunction:
 		return TypeFunction
 	default:
-		return TypeObject
+		panic("bad type flag")
 	}
 }
 
@@ -251,7 +298,9 @@ func (v Value) Get(p string) Value {
 	if vType := v.Type(); !vType.isObject() {
 		panic(&ValueError{"Value.Get", vType})
 	}
-	return makeValue(valueGet(v.ref, p))
+	r := makeValue(valueGet(v.ref, p))
+	runtime.KeepAlive(v)
+	return r
 }
 
 func valueGet(v ref, p string) ref
@@ -262,7 +311,10 @@ func (v Value) Set(p string, x interface{}) {
 	if vType := v.Type(); !vType.isObject() {
 		panic(&ValueError{"Value.Set", vType})
 	}
-	valueSet(v.ref, p, ValueOf(x).ref)
+	xv := ValueOf(x)
+	valueSet(v.ref, p, xv.ref)
+	runtime.KeepAlive(v)
+	runtime.KeepAlive(xv)
 }
 
 func valueSet(v ref, p string, x ref)
@@ -274,6 +326,7 @@ func (v Value) Delete(p string) {
 		panic(&ValueError{"Value.Delete", vType})
 	}
 	valueDelete(v.ref, p)
+	runtime.KeepAlive(v)
 }
 
 func valueDelete(v ref, p string)
@@ -284,7 +337,9 @@ func (v Value) Index(i int) Value {
 	if vType := v.Type(); !vType.isObject() {
 		panic(&ValueError{"Value.Index", vType})
 	}
-	return makeValue(valueIndex(v.ref, i))
+	r := makeValue(valueIndex(v.ref, i))
+	runtime.KeepAlive(v)
+	return r
 }
 
 func valueIndex(v ref, i int) ref
@@ -295,17 +350,23 @@ func (v Value) SetIndex(i int, x interface{}) {
 	if vType := v.Type(); !vType.isObject() {
 		panic(&ValueError{"Value.SetIndex", vType})
 	}
-	valueSetIndex(v.ref, i, ValueOf(x).ref)
+	xv := ValueOf(x)
+	valueSetIndex(v.ref, i, xv.ref)
+	runtime.KeepAlive(v)
+	runtime.KeepAlive(xv)
 }
 
 func valueSetIndex(v ref, i int, x ref)
 
-func makeArgs(args []interface{}) []ref {
-	argVals := make([]ref, len(args))
+func makeArgs(args []interface{}) ([]Value, []ref) {
+	argVals := make([]Value, len(args))
+	argRefs := make([]ref, len(args))
 	for i, arg := range args {
-		argVals[i] = ValueOf(arg).ref
+		v := ValueOf(arg)
+		argVals[i] = v
+		argRefs[i] = v.ref
 	}
-	return argVals
+	return argVals, argRefs
 }
 
 // Length returns the JavaScript property "length" of v.
@@ -314,7 +375,9 @@ func (v Value) Length() int {
 	if vType := v.Type(); !vType.isObject() {
 		panic(&ValueError{"Value.SetIndex", vType})
 	}
-	return valueLength(v.ref)
+	r := valueLength(v.ref)
+	runtime.KeepAlive(v)
+	return r
 }
 
 func valueLength(v ref) int
@@ -323,7 +386,10 @@ func valueLength(v ref) int
 // It panics if v has no method m.
 // The arguments get mapped to JavaScript values according to the ValueOf function.
 func (v Value) Call(m string, args ...interface{}) Value {
-	res, ok := valueCall(v.ref, m, makeArgs(args))
+	argVals, argRefs := makeArgs(args)
+	res, ok := valueCall(v.ref, m, argRefs)
+	runtime.KeepAlive(v)
+	runtime.KeepAlive(argVals)
 	if !ok {
 		if vType := v.Type(); !vType.isObject() { // check here to avoid overhead in success case
 			panic(&ValueError{"Value.Call", vType})
@@ -342,7 +408,10 @@ func valueCall(v ref, m string, args []ref) (ref, bool)
 // It panics if v is not a JavaScript function.
 // The arguments get mapped to JavaScript values according to the ValueOf function.
 func (v Value) Invoke(args ...interface{}) Value {
-	res, ok := valueInvoke(v.ref, makeArgs(args))
+	argVals, argRefs := makeArgs(args)
+	res, ok := valueInvoke(v.ref, argRefs)
+	runtime.KeepAlive(v)
+	runtime.KeepAlive(argVals)
 	if !ok {
 		if vType := v.Type(); vType != TypeFunction { // check here to avoid overhead in success case
 			panic(&ValueError{"Value.Invoke", vType})
@@ -358,7 +427,10 @@ func valueInvoke(v ref, args []ref) (ref, bool)
 // It panics if v is not a JavaScript function.
 // The arguments get mapped to JavaScript values according to the ValueOf function.
 func (v Value) New(args ...interface{}) Value {
-	res, ok := valueNew(v.ref, makeArgs(args))
+	argVals, argRefs := makeArgs(args)
+	res, ok := valueNew(v.ref, argRefs)
+	runtime.KeepAlive(v)
+	runtime.KeepAlive(argVals)
 	if !ok {
 		if vType := v.Type(); vType != TypeFunction { // check here to avoid overhead in success case
 			panic(&ValueError{"Value.Invoke", vType})
@@ -373,7 +445,7 @@ func valueNew(v ref, args []ref) (ref, bool)
 func (v Value) isNumber() bool {
 	return v.ref == valueZero.ref ||
 		v.ref == valueNaN.ref ||
-		(v.ref != valueUndefined.ref && v.ref>>32&nanHead != nanHead)
+		(v.ref != valueUndefined.ref && (v.ref>>32)&nanHead != nanHead)
 }
 
 func (v Value) float(method string) float64 {
@@ -438,15 +510,15 @@ func (v Value) Truthy() bool {
 func (v Value) String() string {
 	switch v.Type() {
 	case TypeString:
-		return jsString(v.ref)
+		return jsString(v)
 	case TypeUndefined:
 		return "<undefined>"
 	case TypeNull:
 		return "<null>"
 	case TypeBoolean:
-		return "<boolean: " + jsString(v.ref) + ">"
+		return "<boolean: " + jsString(v) + ">"
 	case TypeNumber:
-		return "<number: " + jsString(v.ref) + ">"
+		return "<number: " + jsString(v) + ">"
 	case TypeSymbol:
 		return "<symbol>"
 	case TypeObject:
@@ -458,10 +530,12 @@ func (v Value) String() string {
 	}
 }
 
-func jsString(v ref) string {
-	str, length := valuePrepareString(v)
+func jsString(v Value) string {
+	str, length := valuePrepareString(v.ref)
+	runtime.KeepAlive(v)
 	b := make([]byte, length)
 	valueLoadString(str, b)
+	finalizeRef(str)
 	return string(b)
 }
 
@@ -471,7 +545,10 @@ func valueLoadString(v ref, b []byte)
 
 // InstanceOf reports whether v is an instance of type t according to JavaScript's instanceof operator.
 func (v Value) InstanceOf(t Value) bool {
-	return valueInstanceOf(v.ref, t.ref)
+	r := valueInstanceOf(v.ref, t.ref)
+	runtime.KeepAlive(v)
+	runtime.KeepAlive(t)
+	return r
 }
 
 func valueInstanceOf(v ref, t ref) bool
@@ -493,6 +570,7 @@ func (e *ValueError) Error() string {
 // CopyBytesToGo panics if src is not an Uint8Array.
 func CopyBytesToGo(dst []byte, src Value) int {
 	n, ok := copyBytesToGo(dst, src.ref)
+	runtime.KeepAlive(src)
 	if !ok {
 		panic("syscall/js: CopyBytesToGo: expected src to be an Uint8Array")
 	}
@@ -506,6 +584,7 @@ func copyBytesToGo(dst []byte, src ref) (int, bool)
 // CopyBytesToJS panics if dst is not an Uint8Array.
 func CopyBytesToJS(dst Value, src []byte) int {
 	n, ok := copyBytesToJS(dst.ref, src)
+	runtime.KeepAlive(dst)
 	if !ok {
 		panic("syscall/js: CopyBytesToJS: expected dst to be an Uint8Array")
 	}
diff --git a/src/syscall/js/js_js.s b/src/syscall/js/js_js.s
index ab56087c16..47ad6b83e5 100644
--- a/src/syscall/js/js_js.s
+++ b/src/syscall/js/js_js.s
@@ -4,6 +4,10 @@
 
 #include "textflag.h"
 
+TEXT ·finalizeRef(SB), NOSPLIT, $0
+  CallImport
+  RET
+
 TEXT ·stringVal(SB), NOSPLIT, $0
   CallImport
   RET
diff --git a/src/syscall/js/js_test.go b/src/syscall/js/js_test.go
index 10d4364e4c..b5d267c03c 100644
--- a/src/syscall/js/js_test.go
+++ b/src/syscall/js/js_test.go
@@ -18,6 +18,7 @@ package js_test
 import (
 	"fmt"
 	"math"
+	"runtime"
 	"syscall/js"
 	"testing"
 )
@@ -53,7 +54,7 @@ func TestBool(t *testing.T) {
 	if got := dummys.Get("otherBool").Bool(); got != want {
 		t.Errorf("got %#v, want %#v", got, want)
 	}
-	if dummys.Get("someBool") != dummys.Get("someBool") {
+	if !dummys.Get("someBool").Equal(dummys.Get("someBool")) {
 		t.Errorf("same value not equal")
 	}
 }
@@ -68,7 +69,7 @@ func TestString(t *testing.T) {
 	if got := dummys.Get("otherString").String(); got != want {
 		t.Errorf("got %#v, want %#v", got, want)
 	}
-	if dummys.Get("someString") != dummys.Get("someString") {
+	if !dummys.Get("someString").Equal(dummys.Get("someString")) {
 		t.Errorf("same value not equal")
 	}
 
@@ -105,7 +106,7 @@ func TestInt(t *testing.T) {
 	if got := dummys.Get("otherInt").Int(); got != want {
 		t.Errorf("got %#v, want %#v", got, want)
 	}
-	if dummys.Get("someInt") != dummys.Get("someInt") {
+	if !dummys.Get("someInt").Equal(dummys.Get("someInt")) {
 		t.Errorf("same value not equal")
 	}
 	if got := dummys.Get("zero").Int(); got != 0 {
@@ -141,20 +142,20 @@ func TestFloat(t *testing.T) {
 	if got := dummys.Get("otherFloat").Float(); got != want {
 		t.Errorf("got %#v, want %#v", got, want)
 	}
-	if dummys.Get("someFloat") != dummys.Get("someFloat") {
+	if !dummys.Get("someFloat").Equal(dummys.Get("someFloat")) {
 		t.Errorf("same value not equal")
 	}
 }
 
 func TestObject(t *testing.T) {
-	if dummys.Get("someArray") != dummys.Get("someArray") {
+	if !dummys.Get("someArray").Equal(dummys.Get("someArray")) {
 		t.Errorf("same value not equal")
 	}
 
 	// An object and its prototype should not be equal.
 	proto := js.Global().Get("Object").Get("prototype")
 	o := js.Global().Call("eval", "new Object()")
-	if proto == o {
+	if proto.Equal(o) {
 		t.Errorf("object equals to its prototype")
 	}
 }
@@ -167,26 +168,66 @@ func TestFrozenObject(t *testing.T) {
 	}
 }
 
+func TestEqual(t *testing.T) {
+	if !dummys.Get("someFloat").Equal(dummys.Get("someFloat")) {
+		t.Errorf("same float is not equal")
+	}
+	if !dummys.Get("emptyObj").Equal(dummys.Get("emptyObj")) {
+		t.Errorf("same object is not equal")
+	}
+	if dummys.Get("someFloat").Equal(dummys.Get("someInt")) {
+		t.Errorf("different values are not unequal")
+	}
+}
+
 func TestNaN(t *testing.T) {
-	want := js.ValueOf(math.NaN())
-	got := dummys.Get("NaN")
-	if got != want {
-		t.Errorf("got %#v, want %#v", got, want)
+	if !dummys.Get("NaN").IsNaN() {
+		t.Errorf("JS NaN is not NaN")
+	}
+	if !js.ValueOf(math.NaN()).IsNaN() {
+		t.Errorf("Go NaN is not NaN")
+	}
+	if dummys.Get("NaN").Equal(dummys.Get("NaN")) {
+		t.Errorf("NaN is equal to NaN")
 	}
 }
 
 func TestUndefined(t *testing.T) {
-	dummys.Set("test", js.Undefined())
-	if dummys == js.Undefined() || dummys.Get("test") != js.Undefined() || dummys.Get("xyz") != js.Undefined() {
-		t.Errorf("js.Undefined expected")
+	if !js.Undefined().IsUndefined() {
+		t.Errorf("undefined is not undefined")
+	}
+	if !js.Undefined().Equal(js.Undefined()) {
+		t.Errorf("undefined is not equal to undefined")
+	}
+	if dummys.IsUndefined() {
+		t.Errorf("object is undefined")
+	}
+	if js.Undefined().IsNull() {
+		t.Errorf("undefined is null")
+	}
+	if dummys.Set("test", js.Undefined()); !dummys.Get("test").IsUndefined() {
+		t.Errorf("could not set undefined")
 	}
 }
 
 func TestNull(t *testing.T) {
-	dummys.Set("test1", nil)
-	dummys.Set("test2", js.Null())
-	if dummys == js.Null() || dummys.Get("test1") != js.Null() || dummys.Get("test2") != js.Null() {
-		t.Errorf("js.Null expected")
+	if !js.Null().IsNull() {
+		t.Errorf("null is not null")
+	}
+	if !js.Null().Equal(js.Null()) {
+		t.Errorf("null is not equal to null")
+	}
+	if dummys.IsNull() {
+		t.Errorf("object is null")
+	}
+	if js.Null().IsUndefined() {
+		t.Errorf("null is undefined")
+	}
+	if dummys.Set("test", js.Null()); !dummys.Get("test").IsNull() {
+		t.Errorf("could not set null")
+	}
+	if dummys.Set("test", nil); !dummys.Get("test").IsNull() {
+		t.Errorf("could not set nil")
 	}
 }
 
@@ -340,7 +381,7 @@ func TestValueOf(t *testing.T) {
 
 func TestZeroValue(t *testing.T) {
 	var v js.Value
-	if v != js.Undefined() {
+	if !v.IsUndefined() {
 		t.Error("zero js.Value is not js.Undefined()")
 	}
 }
@@ -497,12 +538,24 @@ func TestCopyBytesToJS(t *testing.T) {
 	}
 }
 
+func TestGarbageCollection(t *testing.T) {
+	before := js.JSGo.Get("_values").Length()
+	for i := 0; i < 1000; i++ {
+		_ = js.Global().Get("Object").New().Call("toString").String()
+		runtime.GC()
+	}
+	after := js.JSGo.Get("_values").Length()
+	if after-before > 500 {
+		t.Errorf("garbage collection ineffective")
+	}
+}
+
 // BenchmarkDOM is a simple benchmark which emulates a webapp making DOM operations.
 // It creates a div, and sets its id. Then searches by that id and sets some data.
 // Finally it removes that div.
 func BenchmarkDOM(b *testing.B) {
 	document := js.Global().Get("document")
-	if document == js.Undefined() {
+	if document.IsUndefined() {
 		b.Skip("Not a browser environment. Skipping.")
 	}
 	const data = "someString"