From f9b43044efc65030ed69c3d2c89fd775c7f07d94 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Bartek=20Iwa=C5=84czuk?= <biwanczuk@gmail.com>
Date: Wed, 8 Mar 2023 21:08:54 -0400
Subject: [PATCH] perf: don't add unload event listener (#18082)

This commit changes how "unload" event is handled - before
this commit an event listener was added unconditionally in
the runtime bootstrapping function, which for some reason was
very expensive (0.3ms). Instead of adding an event listener,
a check was added to "dispatchEvent" function that performs
the same action (so it's only called if there's an event dispatched).
---
 ext/web/02_event.js   |  9 +++++++++
 runtime/js/99_main.js | 11 -----------
 2 files changed, 9 insertions(+), 11 deletions(-)

diff --git a/ext/web/02_event.js b/ext/web/02_event.js
index 40731af4ce579e..dd8b4f57b9d94a 100644
--- a/ext/web/02_event.js
+++ b/ext/web/02_event.js
@@ -1056,6 +1056,15 @@ class EventTarget {
       prefix: "Failed to execute 'dispatchEvent' on 'EventTarget'",
     });
 
+    // This is an optimization to avoid creating an event listener
+    // on each startup.
+    // Stores the flag for checking whether unload is dispatched or not.
+    // This prevents the recursive dispatches of unload events.
+    // See https://github.com/denoland/deno/issues/9201.
+    if (event.type === "unload" && self === globalThis_) {
+      globalThis_[SymbolFor("isUnloadDispatched")] = true;
+    }
+
     const { listeners } = self[eventTargetData];
     if (!ReflectHas(listeners, event.type)) {
       setTarget(event, this);
diff --git a/runtime/js/99_main.js b/runtime/js/99_main.js
index e3e7e64d827d37..d043f485e2b803 100644
--- a/runtime/js/99_main.js
+++ b/runtime/js/99_main.js
@@ -30,7 +30,6 @@ const {
   ObjectSetPrototypeOf,
   PromiseResolve,
   Symbol,
-  SymbolFor,
   SymbolIterator,
   PromisePrototypeThen,
   SafeWeakMap,
@@ -404,7 +403,6 @@ function bootstrapMainRuntime(runtimeOptions) {
   if (hasBootstrapped) {
     throw new Error("Worker runtime already bootstrapped");
   }
-
   performance.setTimeOrigin(DateNow());
   globalThis_ = globalThis;
 
@@ -451,15 +449,6 @@ function bootstrapMainRuntime(runtimeOptions) {
 
   core.setPromiseRejectCallback(promiseRejectCallback);
 
-  const isUnloadDispatched = SymbolFor("isUnloadDispatched");
-  // Stores the flag for checking whether unload is dispatched or not.
-  // This prevents the recursive dispatches of unload events.
-  // See https://github.com/denoland/deno/issues/9201.
-  globalThis[isUnloadDispatched] = false;
-  globalThis.addEventListener("unload", () => {
-    globalThis_[isUnloadDispatched] = true;
-  });
-
   runtimeStart(runtimeOptions);
 
   setNumCpus(runtimeOptions.cpuCount);