fix(parser) Fix memory leak when creating multiple instances (#4095)

* fix: The memory leaking issue when creating multiple HLJS instances

Author: immccn123

CHANGES.md

@@ -29,6 +29,10 @@ Themes:
 
 - Added `Rosé Pine` theme [William Wilkinson][]
 
+Improvements:
+
+- Resolve the memory leak problem when creating multiple Highlight.js instances [Imken][]
+
 CONTRIBUTORS
 
 [Josh Goebel]: https://github.com/joshgoebel
@@ -39,6 +43,7 @@ CONTRIBUTORS
 [srawlins]: https://github.com/srawlins
 [Alvin Joy]: https://github.com/alvinsjoy
 [Aboobacker MK]: https://github.com/tachyons
+[Imken]: https://github.com/immccn123
 
 
 ## Version 11.10.0

src/highlight.js

@@ -819,8 +819,17 @@ const HLJS = function(hljs) {
    * auto-highlights all pre>code elements on the page
    */
   function highlightAll() {
+    function boot() {
+      // if a highlight was requested before DOM was loaded, do now
+      highlightAll();
+    }
+
     // if we are called too early in the loading process
     if (document.readyState === "loading") {
+      // make sure the event listener is only added once
+      if (!wantsHighlight) {
+        window.addEventListener('DOMContentLoaded', boot, false);
+      }
       wantsHighlight = true;
       return;
     }
@@ -829,16 +838,6 @@ const HLJS = function(hljs) {
     blocks.forEach(highlightElement);
   }
 
-  function boot() {
-    // if a highlight was requested before DOM was loaded, do now
-    if (wantsHighlight) highlightAll();
-  }
-
-  // make sure we are in the browser environment
-  if (typeof window !== 'undefined' && window.addEventListener) {
-    window.addEventListener('DOMContentLoaded', boot, false);
-  }
-
   /**
    * Register a language grammar module
    *