Add ESLint

.github/workflows/lint.yml

@@ -0,0 +1,40 @@
+name: Lint
+
+on:
+  pull_request:
+    types: [opened, synchronize]
+    paths:
+      - '**/*.[tj]s'
+      - '**/*.?[tj]s'
+      - 'eslint.config.mjs'
+      - 'tsconfig.json'
+      - 'package.json'
+      - 'package-lock.json'
+      - '.nvmrc'
+      - '.github/workflows/lint.yml'
+
+jobs:
+  lint:
+    runs-on: blacksmith-2vcpu-ubuntu-2404
+    steps:
+      - name: Checkout
+        # 6.0.3
+        uses: actions/checkout@df4cb1c069e1874edd31b4311f1884172cec0e10
+
+      # v6.1.0
+      - name: Setup Node
+        uses: actions/setup-node@395ad3262231945c25e8478fd5baf05154b1d79f
+        with:
+          node-version-file: .nvmrc
+          cache: npm
+          cache-dependency-path: package-lock.json
+
+      - name: Install npm packages
+        run: npm ci
+
+      - name: Lint
+        run: npm run lint
+
+      - name: Tell people how to run the failing check
+        if: failure()
+        run: echo "::error::The lint check failed! To run it locally, run \`npm ci && npm run lint\` from the GitHub-Actions repo root."

eslint.config.mjs

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+import {createRequire} from 'node:module';
+import path from 'node:path';
+import {fileURLToPath} from 'node:url';
+
+import jestConfig from 'eslint-config-expensify/jest';
+import nodeConfig from 'eslint-config-expensify/node';
+import scriptsConfig from 'eslint-config-expensify/scripts';
+import tsConfig from 'eslint-config-expensify/typescript';
+import rulesdir from 'eslint-plugin-rulesdir';
+import {defineConfig, globalIgnores} from 'eslint/config';
+
+const projectRoot = path.dirname(fileURLToPath(import.meta.url));
+const require = createRequire(import.meta.url);
+const expensifyConfigDirectory = path.dirname(require.resolve('eslint-config-expensify'));
+
+// eslint-config-expensify/typescript enables custom Expensify rules (rulesdir/*) such as
+// rulesdir/prefer-at, but the typescript config slice does not register eslint-plugin-rulesdir
+// itself (that wiring lives in eslint-config-expensify/expensify, which we do not use here).
+// Register the plugin and point it at eslint-config-expensify's bundled rule implementations
+// so ESLint can resolve those rules. eslint-plugin-rulesdir is a transitive dependency of
+// eslint-config-expensify; it does not need a direct package.json entry.
+rulesdir.RULES_DIR = path.resolve(expensifyConfigDirectory, 'eslint-plugin-expensify');
+
+export default defineConfig([
+    globalIgnores(['node_modules']),
+    {
+        plugins: {
+            rulesdir,
+        },
+    },
+    ...nodeConfig,
+    ...tsConfig,
+    ...scriptsConfig,
+    ...jestConfig,
+    {
+        files: ['**/*.ts'],
+        languageOptions: {
+            parserOptions: {
+                project: path.resolve(projectRoot, 'tsconfig.json'),
+                projectService: false,
+            },
+        },
+        rules: {
+            // node:test is used instead of Jest in this repo.
+            'jest/no-jest-import': 'off',
+        },
+    },
+]);