diff --git a/eslint.config.mjs b/eslint.config.mjs
index 5227b28048..3d9a50db48 100644
--- a/eslint.config.mjs
+++ b/eslint.config.mjs
@@ -32,7 +32,7 @@ export default defineConfig([
 		}
 	},
 	{
-		files: ["**/*.js", "js/app.mjs", "js/ip_access_control.mjs", "js/releasenotes.mjs", "js/server.mjs", "js/vendor.mjs"],
+		files: ["**/*.js", "js/app.mjs", "js/deprecated.mjs", "js/electron_helper.mjs", "js/ip_access_control.mjs", "js/releasenotes.mjs", "js/server.mjs", "js/vendor.mjs", "tests/**/*.mjs"],
 		languageOptions: {
 			ecmaVersion: "latest",
 			globals: {
@@ -104,11 +104,12 @@ export default defineConfig([
 		}
 	},
 	{
-		files: ["**/*.js", "js/app.mjs", "js/ip_access_control.mjs", "js/releasenotes.mjs", "js/server.mjs", "js/vendor.mjs"],
+		files: ["**/*.js", "js/app.mjs", "js/deprecated.mjs", "js/electron_helper.mjs", "js/ip_access_control.mjs", "js/releasenotes.mjs", "js/server.mjs", "js/vendor.mjs", "tests/**/*.mjs"],
 		ignores: [
 			"clientonly/index.js",
 			"js/logger.js",
-			"tests/**/*.js"
+			"tests/**/*.js",
+			"tests/**/*.mjs"
 		],
 		rules: {"no-console": "error"}
 	},
@@ -118,7 +119,7 @@ export default defineConfig([
 	},
 	{
 		files: ["**/*.mjs"],
-		ignores: ["js/app.mjs", "js/ip_access_control.mjs", "js/releasenotes.mjs", "js/server.mjs", "js/vendor.mjs"],
+		ignores: ["js/app.mjs", "js/deprecated.mjs", "js/electron_helper.mjs", "js/ip_access_control.mjs", "js/releasenotes.mjs", "js/server.mjs", "js/vendor.mjs", "tests/**/*.mjs"],
 		languageOptions: {
 			ecmaVersion: "latest",
 			globals: {
@@ -142,7 +143,7 @@ export default defineConfig([
 		}
 	},
 	{
-		files: ["tests/**/*.js"],
+		files: ["tests/**/*.{js,mjs}"],
 		languageOptions: {
 			globals: {
 				...vitest.environments.env.globals
diff --git a/js/deprecated.js b/js/deprecated.js
deleted file mode 100644
index 8c8ef3fc54..0000000000
--- a/js/deprecated.js
+++ /dev/null
@@ -1,4 +0,0 @@
-module.exports = {
-	configs: [],
-	clock: ["secondsColor"]
-};
diff --git a/js/deprecated.mjs b/js/deprecated.mjs
new file mode 100644
index 0000000000..b9d78a10a4
--- /dev/null
+++ b/js/deprecated.mjs
@@ -0,0 +1,4 @@
+const configs = [];
+const clock = ["secondsColor"];
+
+export { configs, clock };
diff --git a/js/electron.js b/js/electron.js
index 9b88d039f3..770f95026b 100644
--- a/js/electron.js
+++ b/js/electron.js
@@ -4,7 +4,7 @@ require("./alias-resolver");
 
 const electron = require("electron");
 const Log = require("./logger");
-const { applyElectronSwitches } = require("./electron_helper");
+const { applyElectronSwitches } = require("./electron_helper.mjs");
 const { getServerPort } = require("#server_functions");
 
 const corePromise = import("./app.mjs").then(({ default: core }) => core);
diff --git a/js/electron_helper.js b/js/electron_helper.mjs
similarity index 92%
rename from js/electron_helper.js
rename to js/electron_helper.mjs
index eeecd7ef9b..fce8e8c34c 100644
--- a/js/electron_helper.js
+++ b/js/electron_helper.mjs
@@ -1,4 +1,4 @@
-const Log = require("./logger");
+import Log from "./logger.js";
 
 /**
  * Applies Electron command-line switches from config.
@@ -27,4 +27,4 @@ const applyElectronSwitches = (commandLine, electronSwitches) => {
 	}
 };
 
-module.exports = { applyElectronSwitches };
+export { applyElectronSwitches };
diff --git a/js/utils.js b/js/utils.js
index e209526173..769d901119 100644
--- a/js/utils.js
+++ b/js/utils.js
@@ -77,11 +77,11 @@ const getModulePositions = () => {
 
 /**
  * Checks the config for deprecated options and throws a warning in the logs
- * if it encounters one option from the deprecated.js list
+ * if it encounters one option from the deprecated.mjs list
  * @param {object} userConfig The user config
  */
 const checkDeprecatedOptions = (userConfig) => {
-	const deprecated = require(`${global.root_path}/js/deprecated`);
+	const deprecated = require(`${global.root_path}/js/deprecated.mjs`);
 
 	// check for deprecated core options
 	const deprecatedOptions = deprecated.configs;
diff --git a/tests/e2e/helpers/global-setup.mjs b/tests/e2e/helpers/global-setup.mjs
index c6f418ff99..07e46cc04f 100644
--- a/tests/e2e/helpers/global-setup.mjs
+++ b/tests/e2e/helpers/global-setup.mjs
@@ -1,6 +1,6 @@
-import {createRequire} from "node:module";
+import { createRequire } from "node:module";
 
 // Load natively: when Vitest evaluates global-setup.js itself, app.mjs would run inside Vite's module loader.
 const helpers = createRequire(import.meta.url)("./global-setup.js");
 
-export const {getPage, startApplication, stopApplication, getDocument, fixupIndex, restoreIndex} = helpers;
+export const { getPage, startApplication, stopApplication, getDocument, fixupIndex, restoreIndex } = helpers;
diff --git a/tests/unit/classes/deprecated_spec.js b/tests/unit/classes/deprecated_spec.mjs
similarity index 88%
rename from tests/unit/classes/deprecated_spec.js
rename to tests/unit/classes/deprecated_spec.mjs
index 1491be5208..7ebff221d1 100644
--- a/tests/unit/classes/deprecated_spec.js
+++ b/tests/unit/classes/deprecated_spec.mjs
@@ -1,4 +1,4 @@
-const deprecated = require("../../../js/deprecated");
+import * as deprecated from "../../../js/deprecated.mjs";
 
 describe("Deprecated", () => {
 	it("should be an object", () => {
diff --git a/tests/unit/functions/electron_helper_spec.js b/tests/unit/functions/electron_helper_spec.mjs
similarity index 95%
rename from tests/unit/functions/electron_helper_spec.js
rename to tests/unit/functions/electron_helper_spec.mjs
index ff2c09231f..001fa1fd04 100644
--- a/tests/unit/functions/electron_helper_spec.js
+++ b/tests/unit/functions/electron_helper_spec.mjs
@@ -1,5 +1,5 @@
-const Log = require("logger");
-const { applyElectronSwitches } = require("../../../js/electron_helper");
+import Log from "../../../js/logger.js";
+import { applyElectronSwitches } from "../../../js/electron_helper.mjs";
 
 describe("electron switches", () => {
 	let commandLine;
diff --git a/vitest.config.mjs b/vitest.config.mjs
index 91ad620cdd..402de4b39e 100644
--- a/vitest.config.mjs
+++ b/vitest.config.mjs
@@ -60,7 +60,7 @@ export default defineConfig({
 					environment: "node",
 					setupFiles: ["./tests/utils/vitest-setup.js"],
 					include: [
-						"tests/unit/**/*_spec.js",
+						"tests/unit/**/*_spec.{js,mjs}",
 						"tests/unit/defaultmodules/calendar/calendar_fetcher_utils_bad_rrule.js"
 					],
 					testTimeout: 20000,