[localization-plugin] Improve localized asset construction performance by 50% (#4862)

* [localization-plugin] Improve localized asset construction performance

* [localization-plugin] Revert reverse iteration

---------

Co-authored-by: David Michon <dmichon-msft@users.noreply.github.com>

common/changes/@rushstack/webpack5-localization-plugin/localization-plugin-perf_2024-08-06-01-24.json

@@ -0,0 +1,10 @@
+{
+  "changes": [
+    {
+      "packageName": "@rushstack/webpack5-localization-plugin",
+      "comment": "Improve performance of localized asset reconstruction.",
+      "type": "patch"
+    }
+  ],
+  "packageName": "@rushstack/webpack5-localization-plugin"
+}

webpack/webpack5-localization-plugin/src/AssetProcessor.ts

@@ -228,19 +228,25 @@ function _reconstructLocalized(
 
         const escapedBackslash: string = element.escapedBackslash || '\\';
 
-        // Replace backslashes with the properly escaped backslash
-        BACKSLASH_REGEX.lastIndex = -1;
-        newValue = newValue.replace(BACKSLASH_REGEX, escapedBackslash);
-
-        // @todo: look into using JSON.parse(...) to get the escaping characters
-        const escapingCharacterSequence: string = escapedBackslash.slice(escapedBackslash.length / 2);
+        if (newValue.includes('\\')) {
+          // The vast majority of localized strings do not contain `\\`, so this check avoids an allocation.
+          // Replace backslashes with the properly escaped backslash
+          BACKSLASH_REGEX.lastIndex = -1;
+          newValue = newValue.replace(BACKSLASH_REGEX, escapedBackslash);
+        }
 
         // Ensure the the quotemark, apostrophe, tab, and newline characters are properly escaped
         ESCAPE_REGEX.lastIndex = -1;
-        newValue = newValue.replace(
-          ESCAPE_REGEX,
-          (match) => `${escapingCharacterSequence}${ESCAPE_MAP.get(match)}`
-        );
+        if (ESCAPE_REGEX.test(newValue)) {
+          // The majority of localized strings do not contain the characters that need to be escaped,
+          // so this check avoids an allocation.
+          // @todo: look into using JSON.parse(...) to get the escaping characters
+          const escapingCharacterSequence: string = escapedBackslash.slice(escapedBackslash.length / 2);
+          newValue = newValue.replace(
+            ESCAPE_REGEX,
+            (match) => `${escapingCharacterSequence}${ESCAPE_MAP.get(match)}`
+          );
+        }
 
         result.replace(element.start, element.end - 1, newValue);
         break;
@@ -305,9 +311,7 @@ function _parseStringToReconstructionSequence(
   const jsonStringifyFormatLocaleForFilenameFn: FormatLocaleForFilenameFn = (locale: string) =>
     JSON.stringify(formatLocaleForFilenameFn(locale));
 
-  let regexResult: RegExpExecArray | null;
-  PLACEHOLDER_REGEX.lastIndex = -1;
-  while ((regexResult = PLACEHOLDER_REGEX.exec(source))) {
+  for (const regexResult of source.matchAll(PLACEHOLDER_REGEX)) {
     const [placeholder, escapedBackslash, elementLabel, placeholderSerialNumber] = regexResult;
     const start: number = regexResult.index;
     const end: number = start + placeholder.length;