js: Add avoidTDZ option

Fixes #7865

4 files changed, 24 insertions, 1 deletions

diff --git a/docs/content/en/hugo-pipes/js.md b/docs/content/en/hugo-pipes/js.md
index 5e9c027d5..07ee560f6 100644
--- a/docs/content/en/hugo-pipes/js.md
+++ b/docs/content/en/hugo-pipes/js.md
@@ -40,6 +40,9 @@ Note that this is meant for small data sets, e.g. config settings. For larger da
 minify [bool]
 : Let `js.Build` handle the minification.
 
+avoidTDZ {{< new-in "0.78.0" >}}
+: 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
+
 target [string]
 : The language target.
   One of: `es5`, `es2015`, `es2016`, `es2017`, `es2018`, `es2019`, `es2020` or `esnext`.
diff --git a/go.sum b/go.sum
index b240a8a77..8470f4778 100644
--- a/go.sum
+++ b/go.sum
@@ -459,6 +459,8 @@ github.com/spf13/afero v1.2.2 h1:5jhuqJyZCZf2JRofRvN/nIFgIWNzPa3/Vz8mYylgbWc=
 github.com/spf13/afero v1.2.2/go.mod h1:9ZxEEn6pIJ8Rxe320qSDBk6AsU0r9pR7Q4OcevTdifk=
 github.com/spf13/afero v1.4.0 h1:jsLTaI1zwYO3vjrzHalkVcIHXTNmdQFepW4OI8H3+x8=
 github.com/spf13/afero v1.4.0/go.mod h1:Ai8FlHk4v/PARR026UzYexafAt9roJ7LcLMAmO6Z93I=
+github.com/spf13/afero v1.4.1 h1:asw9sl74539yqavKaglDM5hFpdJVK0Y5Dr/JOgQ89nQ=
+github.com/spf13/afero v1.4.1/go.mod h1:Ai8FlHk4v/PARR026UzYexafAt9roJ7LcLMAmO6Z93I=
 github.com/spf13/cast v1.3.0 h1:oget//CVOEoFewqQxwr0Ej5yjygnqGkvggSE/gB35Q8=
 github.com/spf13/cast v1.3.0/go.mod h1:Qx5cxh0v+4UWYiBimWS+eyWzqEqokIECu5etghLkUJE=
 github.com/spf13/cast v1.3.1 h1:nFm6S0SMdyzrzcmThSipiEubIDy8WEXKNZ0UOgiRpng=
diff --git a/resources/resource_transformers/js/options.go b/resources/resource_transformers/js/options.go
index 5e74982d3..84a6c1a78 100644
--- a/resources/resource_transformers/js/options.go
+++ b/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{
diff --git a/resources/resource_transformers/js/options_test.go b/resources/resource_transformers/js/options_test.go
index 89d362ab9..abc8091a9 100644
--- a/resources/resource_transformers/js/options_test.go
+++ b/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,
 		},