Enable (and fix) strict ESLint rules

.eslintrc.json

@@ -4,12 +4,12 @@
   },
   "extends": [
     "eslint:recommended",
-    "plugin:@typescript-eslint/eslint-recommended"
+    "plugin:@typescript-eslint/eslint-recommended",
+    "plugin:@typescript-eslint/strict"
   ],
   "parser": "@typescript-eslint/parser",
   "parserOptions": {
-    "ecmaVersion": 11,
-    "sourceType": "module"
+    "project": "./tsconfig.json"
   },
   "plugins": ["@typescript-eslint"],
   "rules": {

build/build-package.ts

@@ -39,7 +39,7 @@ export async function buildAndPack(
   let entry: string;
   let esm: boolean;
   let packageOverrides: Record<string, unknown>;
-  let filesToCopy: ReadonlyArray<string>;
+  let filesToCopy: readonly string[];
 
   if (middlewareToBuild) {
     const middlewareDir = path.join(rootDir, "middlewares", middlewareToBuild);
@@ -239,7 +239,7 @@ async function readJson(path: fsOriginal.PathLike): Promise<any> {
 async function copyStaticFiles({
   filesToCopy,
   distDir,
-}: Readonly<{ filesToCopy: ReadonlyArray<string>; distDir: string }>) {
+}: Readonly<{ filesToCopy: readonly string[]; distDir: string }>) {
   await Promise.all(
     filesToCopy.map(async (source) => {
       const basename = path.basename(source);

build/helpers.ts

@@ -1,7 +1,7 @@
 import * as childProcess from "child_process";
 
 export const npm = (
-  args: ReadonlyArray<string>,
+  args: readonly string[],
   { cwd }: Readonly<{ cwd: string }>
 ): Promise<void> =>
   new Promise((resolve, reject) => {

index.ts

@@ -51,13 +51,11 @@ export interface HelmetOptions {
   xssFilter?: boolean;
 }
 
-interface MiddlewareFunction {
-  (
-    req: IncomingMessage,
-    res: ServerResponse,
-    next: (error?: Error) => void
-  ): void;
-}
+type MiddlewareFunction = (
+  req: IncomingMessage,
+  res: ServerResponse,
+  next: (error?: Error) => void
+) => void;
 
 interface Helmet {
   (options?: Readonly<HelmetOptions>): (
@@ -219,6 +217,9 @@ function getMiddlewareFunctionsFromOptions(
 
 const helmet: Helmet = Object.assign(
   function helmet(options: Readonly<HelmetOptions> = {}) {
+    // People should be able to pass an options object with no prototype,
+    // so we want this optional chaining.
+    // eslint-disable-next-line @typescript-eslint/no-unnecessary-condition
     if (options.constructor?.name === "IncomingMessage") {
       throw new Error(
         "It appears you have done something like `app.use(helmet)`, but it should be `app.use(helmet())`."

middlewares/content-security-policy/index.ts

@@ -1,8 +1,9 @@
 import { IncomingMessage, ServerResponse } from "http";
 
-interface ContentSecurityPolicyDirectiveValueFunction {
-  (req: IncomingMessage, res: ServerResponse): string;
-}
+type ContentSecurityPolicyDirectiveValueFunction = (
+  req: IncomingMessage,
+  res: ServerResponse
+) => string;
 
 type ContentSecurityPolicyDirectiveValue =
   | string

test/helpers.ts

@@ -2,13 +2,15 @@ import { IncomingMessage, ServerResponse } from "http";
 import connect from "connect";
 import supertest from "supertest";
 
-interface MiddlewareFunction {
-  (req: IncomingMessage, res: ServerResponse, next: () => void): void;
-}
+type MiddlewareFunction = (
+  req: IncomingMessage,
+  res: ServerResponse,
+  next: () => void
+) => void;
 
 export async function check(
   middleware: MiddlewareFunction,
-  expectedHeaders: Readonly<{ [headerName: string]: string | null }>
+  expectedHeaders: Readonly<Record<string, string | null>>
 ) {
   const app = connect()
     .use(middleware)