diff --git a/gradle/error-prone.gradle b/gradle/error-prone.gradle
index b834bac630..377b91e3de 100644
--- a/gradle/error-prone.gradle
+++ b/gradle/error-prone.gradle
@@ -17,7 +17,6 @@ tasks.withType(JavaCompile).configureEach {
 				'ReturnValueIgnored', // We don`t want to use ErrorProne's annotations.
 				)
 		error(
-				'ReturnValueIgnored',
 				'SelfAssignment',
 				'StringJoin',
 				'UnnecessarilyFullyQualified',
diff --git a/lib/src/main/resources/com/diffplug/spotless/npm/common-serve.js b/lib/src/main/resources/com/diffplug/spotless/npm/common-serve.js
index edce61465a..1d1409697f 100644
--- a/lib/src/main/resources/com/diffplug/spotless/npm/common-serve.js
+++ b/lib/src/main/resources/com/diffplug/spotless/npm/common-serve.js
@@ -1,65 +1,51 @@
-// this file will be glued to the top of the specific xy-serve.js file
-const debug_serve = false; // set to true for debug log output in node process
-const shutdownServer = require("http-graceful-shutdown");
 const express = require("express");
-const app = express();
-
-app.use(express.json({limit: "50mb"}));
-
 const fs = require("fs");
+const shutdownServer = require("http-graceful-shutdown");
 
-function debugLog() {
-	if (debug_serve) {
-		console.log.apply(this, arguments)
-	}
-}
+// this file will be glued to the top of the specific xy-serve.js file
 
 function getInstanceId() {
-	const args = process.argv.slice(2);
-
-	// Look for the --node-server-instance-id option
-	let instanceId;
+    return process.argv.slice(2).find(arg => arg.startsWith("--node-server-instance-id="))?.split("=")[1]
+        || (() => { throw new Error("Missing --node-server-instance-id argument"); })();
+}
 
-	args.forEach(arg => {
-		if (arg.startsWith('--node-server-instance-id=')) {
-			instanceId = arg.split('=')[1];
-		}
-	});
+function streamOutput(port, instanceId) {
+    debugLog("Server running on port " + port + " for instance " + instanceId);
+    fs.writeFile("server.port.tmp", "" + port, function (err) {
+        if (err) {
+            return console.log(err);
+        } else {
+            fs.rename("server.port.tmp", `server-${instanceId}.port`, function (err) {
+                if (err) {
+                    return console.log(err);
+                }
+            });
+        }
+    });
+}
 
-	// throw if instanceId is not set
-	if (!instanceId) {
-		throw new Error("Missing --node-server-instance-id argument");
-	}
-	return instanceId;
+function debugLog() {
+    if (false) { // set to true for debug log output in node process
+        console.log.apply(this, arguments);
+    }
 }
 
-var listener = app.listen(0, "127.0.0.1", () => {
-	const instanceId = getInstanceId();
-	debugLog("Server running on port " + listener.address().port + " for instance " + instanceId);
-	fs.writeFile("server.port.tmp", "" + listener.address().port, function (err) {
-		if (err) {
-			return console.log(err);
-		} else {
-			fs.rename("server.port.tmp", `server-${instanceId}.port`, function (err) {
-				if (err) {
-					return console.log(err);
-				}
-			}); // try to be as atomic as possible
-		}
-	});
-});
-const shutdown = shutdownServer(listener, {
-	forceExit: false, // let the event loop clear
-	finally: () => debugLog("graceful shutdown finished."),
-});
+const app = express();
+
+app.use(express.json({ limit: "50mb" }));
+
+const server = app.listen(0, "127.0.0.1", () => streamOutput(server.address().port, getInstanceId()));
 
 app.post("/shutdown", (req, res) => {
-	res.status(200).send("Shutting down");
-	setTimeout(async () => {
-		try {
-			await shutdown();
-		} catch (err) {
-			console.error("Error during shutdown:", err);
-		}
-	}, 200);
+    setTimeout(async () => {
+        try {
+            await shutdownServer(server, {
+                forceExit: false, // let the event loop clear
+                finally: () => debugLog("graceful shutdown finished."),
+            })();
+        } catch (err) {
+            console.error("Error during shutdown:", err);
+        }
+    }, 200);
+    res.ok().send("Graceful shutdown.");
 });
diff --git a/lib/src/main/resources/com/diffplug/spotless/npm/prettier-serve.js b/lib/src/main/resources/com/diffplug/spotless/npm/prettier-serve.js
index ac1f26790d..da3121d72f 100644
--- a/lib/src/main/resources/com/diffplug/spotless/npm/prettier-serve.js
+++ b/lib/src/main/resources/com/diffplug/spotless/npm/prettier-serve.js
@@ -22,16 +22,13 @@ app.post("/prettier/config-options", (req, res) => {
 
 app.post("/prettier/format", async (req, res) => {
 	const format_data = req.body;
-
-	let formatted_file_content = "";
 	try {
-		formatted_file_content = await prettierFormat(format_data.file_content, format_data.config_options);
+		res.set("Content-Type", "text/plain");
+		res.send(await prettierFormat(format_data.file_content, format_data.config_options));
 	} catch(err) {
 		res.status(500).send("Error while formatting: " + err);
 		return;
 	}
-	res.set("Content-Type", "text/plain");
-	res.send(formatted_file_content);
 });
 
 const prettierFormat = async function(file_content, config_options) {
@@ -62,7 +59,7 @@ const mergeConfigOptions = function(resolved_config_options, config_options) {
 const extend = function() {
 	// Variables
 	const extended = {};
-	let i = 0;
+	const i = 0;
 	const length = arguments.length;
 
 	// Merge the object into the extended object
diff --git a/lib/src/main/resources/com/diffplug/spotless/npm/tsfmt-serve.js b/lib/src/main/resources/com/diffplug/spotless/npm/tsfmt-serve.js
index ffbb36c8a4..9c3c4f29b0 100644
--- a/lib/src/main/resources/com/diffplug/spotless/npm/tsfmt-serve.js
+++ b/lib/src/main/resources/com/diffplug/spotless/npm/tsfmt-serve.js
@@ -1,23 +1,7 @@
 const tsfmt = require("typescript-formatter");
 
 app.post("/tsfmt/format", (req, res) => {
-	var format_data = req.body;
-	tsfmt.processString("spotless-format-string.ts", format_data.file_content, format_data.config_options).then(resultMap => {
-		/*
-        export interface ResultMap {
-            [fileName: string]: Result;
-        }
-
-        export interface Result {
-            fileName: string;
-            settings: ts.FormatCodeSettings | null;
-            message: string;
-            error: boolean;
-            src: string;
-            dest: string;
-        }
-        */
-		// result contains 'message' (String), 'error' (boolean), 'dest' (String) => formatted
+	tsfmt.processString("spotless-format-string.ts", req.body.file_content, req.body.config_options).then(resultMap => {
 		if (resultMap.error !== undefined && resultMap.error) {
 			res.status(400).send(resultMap.message);
 			return;
diff --git a/plugin-maven/src/test/java/com/diffplug/spotless/maven/npm/NpmTestsWithDynamicallyInstalledNpmInstallationTest.java b/plugin-maven/src/test/java/com/diffplug/spotless/maven/npm/NpmTestsWithDynamicallyInstalledNpmInstallationTest.java
index 78830bf09d..eeb6929f3b 100644
--- a/plugin-maven/src/test/java/com/diffplug/spotless/maven/npm/NpmTestsWithDynamicallyInstalledNpmInstallationTest.java
+++ b/plugin-maven/src/test/java/com/diffplug/spotless/maven/npm/NpmTestsWithDynamicallyInstalledNpmInstallationTest.java
@@ -1,5 +1,5 @@
 /*
- * Copyright 2023 DiffPlug
+ * Copyright 2023-2025 DiffPlug
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -25,6 +25,8 @@
 
 public class NpmTestsWithDynamicallyInstalledNpmInstallationTest extends MavenIntegrationHarness {
 
+	public static final String TS = "typescript";
+
 	@Test
 	void useDownloadedNpmInstallation() throws Exception {
 		writePomWithPrettierSteps(
@@ -34,15 +36,11 @@ void useDownloadedNpmInstallation() throws Exception {
 				"    <npmExecutable>" + installedNpmPath() + "</npmExecutable>",
 				"</prettier>");
 
-		String kind = "typescript";
-		String suffix = "ts";
-		String configPath = ".prettierrc.yml";
-		setFile(configPath).toResource("npm/prettier/filetypes/" + kind + "/" + ".prettierrc.yml");
-		String path = "src/main/" + kind + "/test." + suffix;
-		setFile(path).toResource("npm/prettier/filetypes/" + kind + "/" + kind + ".dirty");
-
+		String path = "src/main/" + TS + "/test.ts";
+		setFile(path).toResource("npm/prettier/filetypes/" + TS + "/" + TS + ".dirty");
+		setFile(".prettierrc.yml").toResource("npm/prettier/filetypes/" + TS + "/" + ".prettierrc.yml");
 		mavenRunner().withArguments(installNpmMavenGoal(), "spotless:apply").runNoError();
-		assertFile(path).sameAsResource("npm/prettier/filetypes/" + kind + "/" + kind + ".clean");
+		assertFile(path).sameAsResource("npm/prettier/filetypes/" + TS + "/" + TS + ".clean");
 	}
 
 }
diff --git a/testlib/src/main/resources/clang/example.js b/testlib/src/main/resources/clang/example.js
index 08ebafc1a2..99b023d78c 100644
--- a/testlib/src/main/resources/clang/example.js
+++ b/testlib/src/main/resources/clang/example.js
@@ -1,4 +1,4 @@
-var numbers=[1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20,
+const numbers=[1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20,
 ];
 
 const p = {
@@ -10,9 +10,9 @@ const p = {
 const str = "Hello, world!"
 ;
 
-var str2=str.charAt(3)+str[0];
+const str2=str.charAt(3)+str[0];
 
-var multilinestr = "Hello \
+const multilinestr = "Hello \
 World"
 ;