switch to new ESLint flat config format

And make everything pass the new, stricter checks.

eslint.config.mjs

@@ -0,0 +1,174 @@
+import js from '@eslint/js';
+import ts from 'typescript-eslint';
+
+import jsoncParser from 'jsonc-eslint-parser';
+
+import prettierPlugin from 'eslint-plugin-prettier';
+import importPlugin from 'eslint-plugin-import';
+
+import prettierConfig from 'eslint-config-prettier';
+
+import globals from 'globals';
+
+/* __dirname is not available, so fake it */
+import path from 'node:path';
+import { fileURLToPath } from 'node:url';
+
+const __filename = fileURLToPath(import.meta.url);
+const __dirname = path.dirname(__filename);
+
+/* eslint-config-airbnb-typescript is not yet compatible with flat config */
+import airbnbBaseConfig from 'eslint-config-airbnb-typescript/base.js'; /* eslint-disable-line import/extensions */
+import { FlatCompat } from '@eslint/eslintrc';
+
+const compat = new FlatCompat({
+  baseDirectory: __dirname,
+});
+
+/* convert eslintrc style to flat config */
+const airbnbCompat = compat.config(airbnbBaseConfig);
+
+/* tries to include a conflicting plugin by default and is missing this one */
+airbnbCompat[0].plugins = {
+  import: importPlugin,
+};
+
+export default ts.config(
+  {
+    ignores: ['frontend/**', 'lib/**', 'node_modules/**', 'dist/**'],
+  },
+  js.configs.recommended,
+  ...ts.configs.strictTypeChecked,
+  ...airbnbCompat,
+  prettierConfig,
+  {
+    /* all JS/TS files */
+    files: ['**/*.{js,mjs,cjs,ts}'],
+    plugins: {
+      prettier: prettierPlugin,
+      import: importPlugin,
+    },
+    rules: {
+      'prettier/prettier': ['error'],
+      /* allow names that start with _ to be unused */
+      '@typescript-eslint/no-unused-vars': [
+        'error',
+        {
+          args: 'all',
+          argsIgnorePattern: '^_',
+          caughtErrors: 'all',
+          caughtErrorsIgnorePattern: '^_',
+          destructuredArrayIgnorePattern: '^_',
+          varsIgnorePattern: '^_',
+        },
+      ],
+    },
+  },
+  {
+    /* non-TypeScript JS files */
+    files: ['**/*.{js,mjs,cjs,json}'],
+    /* disable the TypeScript-specific rules from strictTypeChecked */
+    ...ts.configs.disableTypeChecked,
+  },
+  {
+    /* TypeScript files */
+    files: ['**/*.ts'],
+    languageOptions: {
+      parser: ts.parser,
+      parserOptions: {
+        /* use closest tsconfig.json */
+        project: true,
+        tsconfigRootDir: __dirname,
+      },
+    },
+    plugins: {
+      '@typescript-eslint': ts.plugin,
+    },
+    settings: {
+      'import/resolver': {
+        /* import doesn't seem to check .ts files without this */
+        typescript: {},
+      },
+    },
+    rules: {
+      /* only enable these for TypeScript files */
+      ...ts.configs.stylisticTypeChecked[0].rules,
+      '@typescript-eslint/ban-ts-comment': 'off',
+      /* avoid spurious errors */
+      '@typescript-eslint/no-base-to-string': [
+        'error',
+        {
+          ignoredTypeNames: ['Error', 'ErrnoException'],
+        },
+      ],
+      /* use @ts-expect-error instead of @ts-ignore */
+      '@typescript-eslint/prefer-ts-expect-error': 'error',
+      /* too many errors with this one */
+      '@typescript-eslint/no-explicit-any': 'off',
+    },
+  },
+  {
+    /* check syntax of JSON files */
+    files: ['**/*.json'],
+    languageOptions: {
+      parser: jsoncParser,
+    },
+  },
+  {
+    files: ['services/**/*.{js,ts}', 'tools/*.js', 'eslint.config.{js,cjs,mjs}', 'webpack.config.js'],
+    /* all of these use node */
+    languageOptions: {
+      globals: {
+        ...globals.node,
+      },
+    },
+  },
+  {
+    files: ['services/**/*.{js,ts}'],
+    rules: {
+      /* logging to console is fine */
+      'no-console': 'off',
+      /* no dev dependencies in shipped code */
+      'import/no-extraneous-dependencies': [
+        'error',
+        {
+          devDependencies: false,
+          packageDir: __dirname,
+        },
+      ],
+    },
+  },
+  {
+    files: ['tools/*.js'],
+    rules: {
+      /* not sure how to tell it these aren't modules */
+      '@typescript-eslint/no-var-requires': 'off',
+    },
+  },
+  {
+    files: ['*.config.{js,cjs,mjs}'],
+    rules: {
+      /* obviously we need dev dependencies */
+      'import/no-extraneous-dependencies': [
+        'error',
+        {
+          devDependencies: true,
+          packageDir: __dirname,
+        },
+      ],
+    },
+  },
+  {
+    files: ['eslint.config.{js,cjs,mjs}'],
+    rules: {
+      /* necessary for flat config */
+      'import/no-default-export': 'off',
+    },
+  },
+  {
+    files: ['webpack.config.js'],
+    rules: {
+      '@typescript-eslint/no-var-requires': 'off',
+    },
+  },
+);