syscall.js: add Value.InstanceOf

Change-Id: Icf56188fdb2b8ce6789830a35608203fdb9a3df6
Reviewed-on: https://go-review.googlesource.com/120560Reviewed-by: Paul Jolly <paul@myitcv.org.uk>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

misc/wasm/wasm_exec.js

@@ -286,6 +286,11 @@
 						loadSlice(sp + 16).set(str);
 					},
 
+					// func valueInstanceOf(v ref, t ref) bool
+					"syscall/js.valueInstanceOf": (sp) => {
+						mem().setUint8(sp + 16, loadValue(sp + 8) instanceof loadValue(sp + 12));
+					},
+
 					"debug": (value) => {
 						console.log(value);
 					},

src/cmd/vet/all/whitelist/wasm.txt

@@ -31,3 +31,4 @@ syscall/js/js_js.s: [wasm] valueInt: RET without writing to 8-byte ret+8(FP)
 syscall/js/js_js.s: [wasm] valueBool: RET without writing to 1-byte ret+8(FP)
 syscall/js/js_js.s: [wasm] valueLength: RET without writing to 8-byte ret+8(FP)
 syscall/js/js_js.s: [wasm] valuePrepareString: RET without writing to 4-byte ret+8(FP)
+syscall/js/js_js.s: [wasm] valueInstanceOf: RET without writing to 1-byte ret+8(FP)

src/syscall/js/js.go

@@ -227,3 +227,10 @@ func (v Value) String() string {
 func valuePrepareString(v ref) (ref, int)
 
 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)
+}
+
+func valueInstanceOf(v ref, t ref) bool

src/syscall/js/js_js.s

@@ -71,3 +71,7 @@ TEXT ·valuePrepareString(SB), NOSPLIT, $0
 TEXT ·valueLoadString(SB), NOSPLIT, $0
   CallImport
   RET
+
+TEXT ·valueInstanceOf(SB), NOSPLIT, $0
+  CallImport
+  RET

src/syscall/js/js_test.go

@@ -146,6 +146,16 @@ func TestNew(t *testing.T) {
 	}
 }
 
+func TestInstanceOf(t *testing.T) {
+	someArray := js.Global.Get("Array").New()
+	if got, want := someArray.InstanceOf(js.Global.Get("Array")), true; got != want {
+		t.Errorf("got %#v, want %#v", got, want)
+	}
+	if got, want := someArray.InstanceOf(js.Global.Get("Function")), false; got != want {
+		t.Errorf("got %#v, want %#v", got, want)
+	}
+}
+
 func TestCallback(t *testing.T) {
 	c := make(chan struct{})
 	cb := js.NewCallback(func(args []js.Value) {