jsutil: Use a 'writable' TypedArray on go2cpp

internal/jsutil/buf_js.go

@@ -18,11 +18,18 @@ import (
 	"syscall/js"
 )
 
+// isTypedArrayWritable represents whether TypedArray is writable or not.
+// TypedArray's properties are not writable in the Web standard, but are writable with go2cpp.
+// This enables to avoid unnecessary allocations of js.Value.
+var isTypedArrayWritable = js.Global().Get("go2cpp").Truthy()
+
 // temporaryBuffer is a temporary buffer used at gl.readPixels or gl.texSubImage2D.
 // The read data is converted to Go's byte slice as soon as possible.
 // To avoid often allocating ArrayBuffer, reuse the buffer whenever possible.
 var temporaryBuffer = js.Global().Get("ArrayBuffer").New(16)
 
+var uint8ArrayObj js.Value
+
 func TemporaryUint8Array(byteLength int) js.Value {
 	if bufl := temporaryBuffer.Get("byteLength").Int(); bufl < byteLength {
 		for bufl < byteLength {
@@ -30,5 +37,15 @@ func TemporaryUint8Array(byteLength int) js.Value {
 		}
 		temporaryBuffer = js.Global().Get("ArrayBuffer").New(bufl)
 	}
+	if isTypedArrayWritable {
+		if uint8ArrayObj.IsUndefined() {
+			uint8ArrayObj = js.Global().Get("Uint8Array").New()
+			println("h")
+		}
+		uint8ArrayObj.Set("buffer", temporaryBuffer)
+		uint8ArrayObj.Set("byteOffset", 0)
+		uint8ArrayObj.Set("byteLength", byteLength)
+		return uint8ArrayObj
+	}
 	return js.Global().Get("Uint8Array").New(temporaryBuffer, 0, byteLength)
 }