Add flag to run event handlers sequentially

src/UserConfig.js

@@ -251,6 +251,10 @@ class UserConfig {
 		return this.events.emit(eventName, ...args);
 	}
 
+	setEventEmitterMode(mode) {
+		this.events.setHandlerMode(mode);
+	}
+
 	/*
 	 * Universal getters
 	 */

src/Util/AsyncEventEmitter.js

@@ -5,6 +5,8 @@ import { EventEmitter } from "node:events";
  * It can be used for time measurements during a build.
  */
 class AsyncEventEmitter extends EventEmitter {
+	#handlerMode = "parallel";
+
 	// TypeScript slop
 	constructor(...args) {
 		super(...args);
@@ -22,7 +24,20 @@ class AsyncEventEmitter extends EventEmitter {
 			return [];
 		}
 
-		return Promise.all(listeners.map((listener) => listener.apply(this, args)));
+		if (this.#handlerMode == "sequential") {
+			const result = [];
+			for (const listener of listeners) {
+				const returnValue = await listener.apply(this, args);
+				result.push(returnValue);
+			}
+			return result;
+		} else {
+			return Promise.all(
+				listeners.map((listener) => {
+					return listener.apply(this, args);
+				}),
+			);
+		}
 	}
 
 	/**
@@ -51,6 +66,10 @@ class AsyncEventEmitter extends EventEmitter {
 
 		return this.emit.call(this, type, ...argsMap);
 	}
+
+	setHandlerMode(mode) {
+		this.#handlerMode = mode;
+	}
 }
 
 export default AsyncEventEmitter;

test/EleventyTest.js

@@ -816,6 +816,33 @@ test("eleventy.before and eleventy.after Event Arguments, directories", async (t
   let results = await elev.toJSON();
 });
 
+test("eleventy.after fires sequentially setting eventEmitterMode 'sequential'", async (t) => {
+  let reachFirst;
+  const firstReached = new Promise(resolve => reachFirst = resolve)
+  let next;
+  const firstResult = new Promise(resolve => next = resolve)
+  let secondCalled = false;
+  let elev = new Eleventy("./test/noop/", "./test/noop/_site", {
+    config: function (eleventyConfig) {
+      eleventyConfig.setEventEmitterMode('sequential')
+      eleventyConfig.on("eleventy.after", arg => {
+        reachFirst()
+        return firstResult;
+      })
+      eleventyConfig.on("eleventy.after", arg => {
+        secondCalled = true;
+      })
+    },
+  });
+  const resultPromise = elev.toJSON();
+  await firstReached;
+  t.is(secondCalled, false)
+  next()
+  await 'microtask'
+  t.is(secondCalled, true)
+  await resultPromise;
+})
+
 test("setInputDirectory config method #1503", async (t) => {
   t.plan(5);
   let elev = new Eleventy("./test/noop/", "./test/noop/_site", {