From 0dd426233d17d24238d3a439eeed802ef41edd86 Mon Sep 17 00:00:00 2001
From: Nedyalko Andreev <n@andreev.sh>
Date: Mon, 3 Jun 2019 11:50:37 +0300
Subject: [PATCH] Reuse the parsed core-js library across VUs

---
 js/lib/lib.go             | 22 +++++++++++++++++-----
 release notes/upcoming.md |  4 ++++
 2 files changed, 21 insertions(+), 5 deletions(-)

diff --git a/js/lib/lib.go b/js/lib/lib.go
index 4d350dea818..f3c6adad30f 100644
--- a/js/lib/lib.go
+++ b/js/lib/lib.go
@@ -23,14 +23,26 @@
 package lib
 
 import (
+	"sync"
+
 	"github.com/GeertJohan/go.rice"
 	"github.com/dop251/goja"
 )
 
+//nolint:gochecknoglobals
+var (
+	once   sync.Once
+	coreJs *goja.Program
+)
+
 func GetCoreJS() *goja.Program {
-	return goja.MustCompile(
-		"core-js/shim.min.js",
-		rice.MustFindBox("core-js").MustString("shim.min.js"),
-		true,
-	)
+	once.Do(func() {
+		coreJs = goja.MustCompile(
+			"core-js/shim.min.js",
+			rice.MustFindBox("core-js").MustString("shim.min.js"),
+			true,
+		)
+	})
+
+	return coreJs
 }
diff --git a/release notes/upcoming.md b/release notes/upcoming.md
index bb1607bee23..76edc368d20 100644
--- a/release notes/upcoming.md	
+++ b/release notes/upcoming.md	
@@ -15,3 +15,7 @@ Now all http methods have an additional param called `compression` that will mak
   - windows: work with paths starting with `/` or `\` as absolute from the current drive
 
 * JS: Correctly always set `response.url` to be the URL that was ultimately fetched (i.e. after any potential redirects), even if there were non http errors. (#990)
+
+## Internals
+
+* JS: VU initialization time and memory usage has been significantly decreased by caching the parsed version of the core-js library. Thanks, @matlockx! (#1038)
\ No newline at end of file