js: Add avoidTDZ option

Fixes #7865

resources/resource_transformers/js/options.go

@@ -70,6 +70,16 @@ type Options struct {
 	// What to use instead of React.Fragment.
 	JSXFragment string
 
+	// There is/was a bug in WebKit with severe performance issue with the tracking
+	// of TDZ checks in JavaScriptCore.
+	//
+	// Enabling this flag removes the TDZ and `const` assignment checks and
+	// may improve performance of larger JS codebases until the WebKit fix
+	// is in widespread use.
+	//
+	// See https://bugs.webkit.org/show_bug.cgi?id=199866
+	AvoidTDZ bool
+
 	mediaType  media.Type
 	outDir     string
 	contents   string
@@ -339,6 +349,8 @@ func toBuildOptions(opts Options) (buildOptions api.BuildOptions, err error) {
 		JSXFactory:  opts.JSXFactory,
 		JSXFragment: opts.JSXFragment,
 
+		AvoidTDZ: opts.AvoidTDZ,
+
 		Tsconfig: opts.tsConfig,
 
 		Stdin: &api.StdinOptions{

resources/resource_transformers/js/options_test.go

@@ -54,7 +54,12 @@ func TestToBuildOptions(t *testing.T) {
 	})
 
 	opts, err = toBuildOptions(Options{
-		Target: "es2018", Format: "cjs", Minify: true, mediaType: media.JavascriptType})
+		Target:    "es2018",
+		Format:    "cjs",
+		Minify:    true,
+		mediaType: media.JavascriptType,
+		AvoidTDZ:  true,
+	})
 	c.Assert(err, qt.IsNil)
 	c.Assert(opts, qt.DeepEquals, api.BuildOptions{
 		Bundle:            true,
@@ -63,6 +68,7 @@ func TestToBuildOptions(t *testing.T) {
 		MinifyIdentifiers: true,
 		MinifySyntax:      true,
 		MinifyWhitespace:  true,
+		AvoidTDZ:          true,
 		Stdin: &api.StdinOptions{
 			Loader: api.LoaderJS,
 		},