chore: upgrade eslint backend to 9.0 (#1259)

Author: TCMeldrum

apps/backend/.eslintignore

@@ -0,0 +1,123 @@
+// For some reason ESLint's own types are not resolved correctly :)
+/* eslint-disable import/no-unresolved */
+import eslint from '@eslint/js';
+import tsParser from '@typescript-eslint/parser';
+import { defineConfig } from 'eslint/config';
+import importPlugin from 'eslint-plugin-import';
+import jest from 'eslint-plugin-jest';
+import prettier from 'eslint-plugin-prettier';
+import unusedImports from 'eslint-plugin-unused-imports';
+import globals from 'globals';
+import tseslint from 'typescript-eslint';
+
+export default defineConfig([
+  {
+    ignores: [
+      'build/*',
+      'coverage/*',
+      '**/*.d.ts', // data definition files
+      'src/public/', //3rd party libs
+      'src/types/',
+      'openapi.yaml', //# auto-generated REST client for the STFC UserOfficeWebService
+      'generated/',
+      // node_modules is ignored by default
+    ],
+  },
+  eslint.configs.recommended,
+  tseslint.configs.recommended,
+  importPlugin.flatConfigs.recommended,
+  importPlugin.flatConfigs.typescript,
+  {
+    files: ['**/*.{ts,js}'],
+    languageOptions: {
+      parser: tsParser,
+      parserOptions: {
+        ecmaVersion: 'es2018',
+        sourceType: 'module',
+      },
+      globals: { ...globals.node },
+    },
+    plugins: {
+      'unused-imports': unusedImports,
+      prettier,
+      jest,
+    },
+    rules: {
+      semi: ['error', 'always'],
+      quotes: ['error', 'single'],
+      'no-console': 'error', // prefer `duo-logger` over console
+      'prettier/prettier': 'error',
+      'import/order': [
+        'error',
+        {
+          groups: [
+            'builtin',
+            'external',
+            'internal',
+            ['parent', 'sibling'],
+            'index',
+          ],
+          // distinctGroup: true,
+          pathGroups: [
+            {
+              pattern: '@src/**',
+              group: 'internal',
+              position: 'after',
+            },
+          ],
+          pathGroupsExcludedImportTypes: ['builtin'],
+          'newlines-between': 'always',
+          alphabetize: {
+            order: 'asc',
+            caseInsensitive: true,
+          },
+        },
+      ],
+
+      // TypeScript rules
+      '@typescript-eslint/explicit-module-boundary-types': 'off',
+      '@typescript-eslint/naming-convention': [
+        'error',
+        {
+          selector: 'variable',
+          format: ['camelCase', 'PascalCase', 'UPPER_CASE'],
+          leadingUnderscore: 'allow',
+          trailingUnderscore: 'allow',
+        },
+        {
+          selector: 'function',
+          format: ['PascalCase', 'camelCase'],
+        },
+      ],
+      '@typescript-eslint/no-explicit-any': 'warn',
+      '@typescript-eslint/no-inferrable-types': [
+        'warn',
+        {
+          ignoreParameters: true,
+        },
+      ],
+      '@typescript-eslint/no-unused-vars': 'warn',
+      '@typescript-eslint/ban-ts-comment': 'warn',
+
+      // Other style rules
+      'padding-line-between-statements': [
+        'error',
+        { blankLine: 'always', prev: '*', next: 'return' },
+      ],
+
+      // Handle unused imports
+      'unused-imports/no-unused-imports': 'error',
+      //TS already handles undef variables
+      'no-undef': 'off',
+      'no-unused-vars': 'off',
+      'no-unused-expressions': 'off',
+      //We use these 2 rules in some places intentionally
+      'no-prototype-builtins': 'off',
+      'no-case-declarations': 'off',
+      '@typescript-eslint/no-unused-expressions': [
+        'error',
+        { allowShortCircuit: true, allowTernary: true },
+      ],
+    },
+  },
+]);