From b912184a2c01ca36a412a99b0a03636b91af9501 Mon Sep 17 00:00:00 2001
From: Matt <mfalzon@atlassian.com>
Date: Thu, 5 Jun 2025 11:27:23 -0400
Subject: [PATCH 1/2] improve arrow function support

---
 .../packagers/js/src/ScopeHoistingPackager.js | 15 ++++++++++---
 packages/packagers/js/src/helpers.js          | 21 +++++++++++--------
 2 files changed, 24 insertions(+), 12 deletions(-)

diff --git a/packages/packagers/js/src/ScopeHoistingPackager.js b/packages/packagers/js/src/ScopeHoistingPackager.js
index c07bfbfe36..8105659ce1 100644
--- a/packages/packagers/js/src/ScopeHoistingPackager.js
+++ b/packages/packagers/js/src/ScopeHoistingPackager.js
@@ -104,6 +104,7 @@ export class ScopeHoistingPackager {
   externalAssets: Set<Asset> = new Set();
   forceSkipWrapAssets: Array<string> = [];
   logger: PluginLogger;
+  supportsArrowFunctions: boolean;
 
   constructor(
     options: PluginOptions,
@@ -121,6 +122,10 @@ export class ScopeHoistingPackager {
     this.useAsyncBundleRuntime = useAsyncBundleRuntime;
     this.forceSkipWrapAssets = forceSkipWrapAssets ?? [];
     this.logger = logger;
+    this.supportsArrowFunctions = this.bundle.env.supports(
+      'arrow-functions',
+      true,
+    );
 
     let OutputFormat = OUTPUT_FORMATS[this.bundle.env.outputFormat];
     this.outputFormat = new OutputFormat(this);
@@ -341,7 +346,7 @@ export class ScopeHoistingPackager {
 
     let params = [
       JSON.stringify(this.bundle.publicId),
-      fnExpr(this.bundle.env, [], [codeToRun]),
+      fnExpr(this.supportsArrowFunctions, [], [codeToRun]),
       `${JSON.stringify(deps)}${
         conditions.length > 0
           ? `.concat([${conditions
@@ -717,9 +722,13 @@ export class ScopeHoistingPackager {
       sourceMap?.offsetLines(1, 1);
       lineCount++;
 
+      const args = '(module, exports)';
+      const func = this.supportsArrowFunctions
+        ? `${args} =>`
+        : `function${args}`;
       code = `parcelRegister(${JSON.stringify(
         this.bundleGraph.getAssetPublicId(asset),
-      )}, function(module, exports) {
+      )}, ${func} {
 ${code}
 });
 `;
@@ -1552,7 +1561,7 @@ ${code}
   }
 
   buildFunctionExpression(args: Array<string>, expr: string): string {
-    return this.bundle.env.supports('arrow-functions', true)
+    return this.supportsArrowFunctions
       ? `(${args.join(', ')}) => ${expr}`
       : `function (${args.join(', ')}) { return ${expr}; }`;
   }
diff --git a/packages/packagers/js/src/helpers.js b/packages/packagers/js/src/helpers.js
index 1ece9a2971..c0ffaf8ade 100644
--- a/packages/packagers/js/src/helpers.js
+++ b/packages/packagers/js/src/helpers.js
@@ -36,20 +36,22 @@ var parcelRegister = parcelRequire.register;
 `;
 
 export const fnExpr = (
-  env: Environment,
+  supportsArrowFunctions: boolean,
   params: Array<string>,
   body: Array<string>,
 ): string => {
   let block = `{ ${body.join(' ')} }`;
 
-  if (env.supports('arrow-functions')) {
+  if (supportsArrowFunctions) {
     return `(${params.join(', ')}) => ${block}`;
   }
 
   return `function (${params.join(', ')}) ${block}`;
 };
 
-export const bundleQueuePrelude = (env: Environment): string => `
+export const bundleQueuePrelude = (env: Environment): string => {
+  const supportsArrowFunctions = env.supports('arrow-functions');
+  return `
 if (!$parcel$global.lb) {
   // Set of loaded bundles
   $parcel$global.lb = new Set();
@@ -58,14 +60,14 @@ if (!$parcel$global.lb) {
 
   // Register loaded bundle
   $parcel$global.rlb = ${fnExpr(
-    env,
+    supportsArrowFunctions,
     ['bundle'],
     ['$parcel$global.lb.add(bundle);', '$parcel$global.pq();'],
   )}
 
   // Run when ready
   $parcel$global.rwr = ${fnExpr(
-    env,
+    supportsArrowFunctions,
     // b = bundle public id
     // r = run function to execute the bundle entry
     // d = list of dependent bundles this bundle requires before executing
@@ -75,15 +77,15 @@ if (!$parcel$global.lb) {
 
   // Process queue
   $parcel$global.pq = ${fnExpr(
-    env,
+    supportsArrowFunctions,
     [],
     [
       `var runnableEntry = $parcel$global.bq.find(${fnExpr(
-        env,
+        supportsArrowFunctions,
         ['i'],
         [
           `return i.d.every(${fnExpr(
-            env,
+            supportsArrowFunctions,
             ['dep'],
             ['return $parcel$global.lb.has(dep);'],
           )});`,
@@ -91,7 +93,7 @@ if (!$parcel$global.lb) {
       )});`,
       'if (runnableEntry) {',
       `$parcel$global.bq = $parcel$global.bq.filter(${fnExpr(
-        env,
+        supportsArrowFunctions,
         ['i'],
         ['return i.b !== runnableEntry.b;'],
       )});`,
@@ -102,6 +104,7 @@ if (!$parcel$global.lb) {
   )}
 }
 `;
+};
 
 const $parcel$export = `
 function $parcel$export(e, n, v, s) {

From f6ee6cbda8c6a6f6e874ebf57e1f55240a99e86e Mon Sep 17 00:00:00 2001
From: Matt <mfalzon@atlassian.com>
Date: Thu, 5 Jun 2025 11:33:34 -0400
Subject: [PATCH 2/2] add changeset

---
 .changeset/stupid-kiwis-change.md | 5 +++++
 1 file changed, 5 insertions(+)
 create mode 100644 .changeset/stupid-kiwis-change.md

diff --git a/.changeset/stupid-kiwis-change.md b/.changeset/stupid-kiwis-change.md
new file mode 100644
index 0000000000..a722413b9b
--- /dev/null
+++ b/.changeset/stupid-kiwis-change.md
@@ -0,0 +1,5 @@
+---
+'@atlaspack/packager-js': minor
+---
+
+Use arrow functions in `parcelRegister` calls when supported. Reduce calls to `this.bundle.env.supports('arrow-functions', true);`.