js: define Wrapper interface to support external wrappers for JS values

The Callback and TypedArray are the only JavaScript types supported by
the library, thus they are special-cased in a type switch of ValueOf.

Instead, a Wrapper interface is defined to allow external wrapper types
to be handled properly by ValueOf.

src/syscall/js/callback.go

@@ -20,6 +20,8 @@ var (
 	nextCallbackID uint32 = 1
 )
 
+var _ Wrapper = Callback{} // Callback must implement Wrapper
+
 // Callback is a Go function that got wrapped for use as a JavaScript callback.
 type Callback struct {
 	Value // the JavaScript function that queues the callback for execution

src/syscall/js/js.go

@@ -26,11 +26,22 @@ 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).
 const nanHead = 0x7FF80000
 
+// Wrapper is implemented by types that are backed by a JavaScript value.
+type Wrapper interface {
+	// JSValue returns a JavaScript value associated with an object.
+	JSValue() Value
+}
+
 // Value represents a JavaScript value. The zero value is the JavaScript value "undefined".
 type Value struct {
 	ref ref
 }
 
+// JSValue implements Wrapper interface.
+func (v Value) JSValue() Value {
+	return v
+}
+
 func makeValue(v ref) Value {
 	return Value{ref: v}
 }
@@ -105,12 +116,10 @@ func Global() Value {
 //  | map[string]interface{} | new object             |
 func ValueOf(x interface{}) Value {
 	switch x := x.(type) {
-	case Value:
+	case Value: // should precede Wrapper to avoid a loop
 		return x
-	case TypedArray:
-		return x.Value
-	case Callback:
-		return x.Value
+	case Wrapper:
+		return x.JSValue()
 	case nil:
 		return valueNull
 	case bool:

src/syscall/js/typedarray.go

@@ -22,6 +22,8 @@ var (
 	float64Array = Global().Get("Float64Array")
 )
 
+var _ Wrapper = TypedArray{} // TypedArray must implement Wrapper
+
 // TypedArray represents a JavaScript typed array.
 type TypedArray struct {
 	Value