Respect maximum listeners to avoid warning

README.md

@@ -62,6 +62,26 @@ The default options passed to Critical are:
 
 Where `content` and `outputPath` are the arguments passed to [Eleventy transforms](https://www.11ty.dev/docs/config/#transforms) and `outputDir` is the output directory specified in your [Eleventy configuration](https://www.11ty.dev/docs/config/#output-directory).
 
+### Maximum Concurrency
+
+Node.js will print a warning if more than 10 listeners are added for a particular event. This is a useful default that helps finding memory leaks.
+
+So that you don't see warnings when using eleventy-critical-css, the plugin respects the maximum number of listeners. You can use `process.setMaxListeners()` to increase the concurrency of eleventy-critical-css above the default, but be aware that it may be harder to detect memory leaks in your application as a result.
+
+```js
+const criticalCss = require("eleventy-critical-css");
+
+// Increase concurrency to 100
+process.setMaxListeners(100);
+
+// Unlimited concurrency
+process.setMaxListeners(0);
+
+module.exports = function (eleventyConfig) {
+  eleventyConfig.addPlugin(criticalCss);
+};
+```
+
 <!-- Badges -->
 
 [npm-version-src]: https://img.shields.io/npm/v/eleventy-critical-css/latest.svg

index.js

@@ -1,5 +1,42 @@
 const path = require("path");
 
+let maxListeners;
+let currentListeners = 1;
+
+const functionsToProcess = [];
+
+const startProcessing = async () => {
+  if (maxListeners === undefined) {
+    maxListeners = process.getMaxListeners();
+  }
+
+  if (currentListeners < maxListeners) {
+    const entry = functionsToProcess.pop();
+
+    if (entry) {
+      currentListeners++;
+      const [resolve, reject, functionToProcess] = entry;
+
+      functionToProcess()
+        .then(resolve)
+        .catch(reject)
+        .finally(() => {
+          currentListeners--;
+          startProcessing();
+        });
+    }
+  }
+};
+
+const processFunction = async (functionToProcess) => {
+  const promise = new Promise((resolve, reject) => {
+    functionsToProcess.push([resolve, reject, functionToProcess]);
+  });
+
+  startProcessing();
+  return await promise;
+};
+
 module.exports = function (config, options) {
   let critical;
 
@@ -9,16 +46,20 @@ module.exports = function (config, options) {
 
   config.addTransform("critical-css", async function (content, outputPath) {
     if (outputPath && outputPath.endsWith(".html")) {
-      const { html } = await critical.generate({
-        assetPaths: [path.dirname(outputPath)],
-        base: config.dir.output ?? "_site",
-        html: content,
-        inline: true,
-        rebase: ({ originalUrl }) => originalUrl,
-        ...options,
-      });
-
-      return html;
+      const functionToProcess = async () => {
+        const { html } = await critical.generate({
+          assetPaths: [path.dirname(outputPath)],
+          base: config.dir?.output ?? "_site",
+          html: content,
+          inline: true,
+          rebase: ({ originalUrl }) => originalUrl,
+          ...options,
+        });
+
+        return html;
+      };
+
+      return await processFunction(functionToProcess);
     }
 
     return content;