diff --git a/src/ModuleLoader.ts b/src/ModuleLoader.ts
index 3442b5f1061..2891de29418 100644
--- a/src/ModuleLoader.ts
+++ b/src/ModuleLoader.ts
@@ -158,10 +158,14 @@ export class ModuleLoader {
 		return module;
 	}
 
-	public preloadModule(resolvedId: NormalizedResolveIdWithoutDefaults): Promise<ModuleInfo> {
-		return this.fetchModule(this.addDefaultsToResolvedId(resolvedId)!, undefined, false, true).then(
-			module => module.info
+	async preloadModule(resolvedId: NormalizedResolveIdWithoutDefaults): Promise<ModuleInfo> {
+		const module = await this.fetchModule(
+			this.addDefaultsToResolvedId(resolvedId)!,
+			undefined,
+			false,
+			true
 		);
+		return module.info;
 	}
 
 	resolveId = async (
@@ -237,7 +241,11 @@ export class ModuleLoader {
 		);
 	}
 
-	private async addModuleSource(id: string, importer: string | undefined, module: Module) {
+	private async addModuleSource(
+		id: string,
+		importer: string | undefined,
+		module: Module
+	): Promise<void> {
 		timeStart('load modules', 3);
 		let source: string | SourceDescription;
 		try {
@@ -340,7 +348,7 @@ export class ModuleLoader {
 			return existingModule;
 		}
 
-		const module: Module = new Module(
+		const module = new Module(
 			this.graph,
 			id,
 			this.options,
@@ -379,7 +387,7 @@ export class ModuleLoader {
 		module: Module,
 		resolveStaticDependencyPromises: ResolveStaticDependencyPromise[],
 		resolveDynamicDependencyPromises: ResolveDynamicDependencyPromise[]
-	) {
+	): Promise<void> {
 		await Promise.all([
 			this.fetchStaticDependencies(module, resolveStaticDependencyPromises),
 			this.fetchDynamicDependencies(module, resolveDynamicDependencyPromises)
@@ -520,7 +528,11 @@ export class ModuleLoader {
 		);
 	}
 
-	private async handleExistingModule(module: Module, isEntry: boolean, isPreload: boolean) {
+	private async handleExistingModule(
+		module: Module,
+		isEntry: boolean,
+		isPreload: boolean
+	): Promise<void> {
 		const loadPromise = this.moduleLoadPromises.get(module);
 		if (isPreload) {
 			await loadPromise;
@@ -664,7 +676,7 @@ function addChunkNamesToModule(
 	module: Module,
 	{ fileName, name }: UnresolvedModule,
 	isUserDefined: boolean
-) {
+): void {
 	if (fileName !== null) {
 		module.chunkFileNames.add(fileName);
 	} else if (name !== null) {
@@ -681,7 +693,7 @@ function isNotAbsoluteExternal(
 	id: string,
 	source: string,
 	makeAbsoluteExternalsRelative: boolean | 'ifRelativeSource'
-) {
+): boolean {
 	return (
 		makeAbsoluteExternalsRelative === true ||
 		(makeAbsoluteExternalsRelative === 'ifRelativeSource' && isRelative(source)) ||
diff --git a/src/utils/fs.ts b/src/utils/fs.ts
index 01990bd88a3..efab8a79343 100644
--- a/src/utils/fs.ts
+++ b/src/utils/fs.ts
@@ -3,12 +3,13 @@ import { dirname } from './path';
 
 export * from 'fs';
 
-export const readFile = (file: string): Promise<string> =>
-	new Promise<string>((fulfil, reject) =>
+export function readFile(file: string): Promise<string> {
+	return new Promise((fulfil, reject) =>
 		fs.readFile(file, 'utf-8', (err, contents) => (err ? reject(err) : fulfil(contents)))
 	);
+}
 
-function mkdirpath(path: string) {
+function mkdirpath(path: string): void {
 	const dir = dirname(path);
 	try {
 		fs.readdirSync(dir);
@@ -25,7 +26,7 @@ function mkdirpath(path: string) {
 }
 
 export function writeFile(dest: string, data: string | Uint8Array): Promise<void> {
-	return new Promise<void>((fulfil, reject) => {
+	return new Promise((fulfil, reject) => {
 		mkdirpath(dest);
 
 		fs.writeFile(dest, data, err => {
diff --git a/src/utils/path.ts b/src/utils/path.ts
index e855ec294c5..870d591630d 100644
--- a/src/utils/path.ts
+++ b/src/utils/path.ts
@@ -10,7 +10,7 @@ export function isRelative(path: string): boolean {
 }
 
 export function normalize(path: string): string {
-	if (path.indexOf('\\') == -1) return path;
+	if (!path.includes('\\')) return path;
 	return path.replace(/\\/g, '/');
 }
 
diff --git a/src/utils/resolveId.ts b/src/utils/resolveId.ts
index c6a908f926f..c98ea780336 100644
--- a/src/utils/resolveId.ts
+++ b/src/utils/resolveId.ts
@@ -33,7 +33,7 @@ export async function resolveId(
 
 	// external modules (non-entry modules that start with neither '.' or '/')
 	// are skipped at this stage.
-	if (importer !== undefined && !isAbsolute(source) && source[0] !== '.') return null;
+	if (importer !== undefined && !isAbsolute(source) && !source.startsWith('.')) return null;
 
 	// `resolve` processes paths from right to left, prepending them until an
 	// absolute path is created. Absolute importees therefore shortcircuit the
@@ -45,13 +45,12 @@ export async function resolveId(
 	);
 }
 
-function addJsExtensionIfNecessary(file: string, preserveSymlinks: boolean) {
-	let found = findFile(file, preserveSymlinks);
-	if (found) return found;
-	found = findFile(file + '.mjs', preserveSymlinks);
-	if (found) return found;
-	found = findFile(file + '.js', preserveSymlinks);
-	return found;
+function addJsExtensionIfNecessary(file: string, preserveSymlinks: boolean): string | undefined {
+	return (
+		findFile(file, preserveSymlinks) ??
+		findFile(file + '.mjs', preserveSymlinks) ??
+		findFile(file + '.js', preserveSymlinks)
+	);
 }
 
 function findFile(file: string, preserveSymlinks: boolean): string | undefined {
@@ -64,7 +63,7 @@ function findFile(file: string, preserveSymlinks: boolean): string | undefined {
 			const name = basename(file);
 			const files = readdirSync(dirname(file));
 
-			if (files.indexOf(name) !== -1) return file;
+			if (files.includes(name)) return file;
 		}
 	} catch {
 		// suppress