Reduce the chance of mistakes by linting the code

Refs #13

.eslintignore

@@ -0,0 +1,4 @@
+/.dev/
+/dist/
+/node_modules/
+/src/.observablehq/

.eslintrc.json

@@ -0,0 +1,53 @@
+{
+  "parser": "@typescript-eslint/parser",
+  "plugins": ["@typescript-eslint"],
+  "extends": [
+    "eslint:recommended",
+    "plugin:@typescript-eslint/strict-type-checked",
+    "plugin:@typescript-eslint/stylistic-type-checked",
+    "plugin:import/recommended",
+    "plugin:import/typescript",
+    "plugin:markdown/recommended-legacy",
+    "prettier"
+  ],
+  "parserOptions": {
+    "project": "./tsconfig.json"
+  },
+  "rules": {
+    "@typescript-eslint/array-type": ["error", { "default": "generic" }],
+    "@typescript-eslint/consistent-type-imports": ["error", { "fixStyle": "inline-type-imports" }],
+    "@typescript-eslint/no-confusing-void-expression": "off",
+    "@typescript-eslint/no-import-type-side-effects": "error",
+    "@typescript-eslint/prefer-nullish-coalescing": ["error", { "ignoreConditionalTests": true }],
+    "@typescript-eslint/restrict-template-expressions": ["error", { "allowNumber": true }],
+    "@typescript-eslint/strict-boolean-expressions": "error",
+    "import/no-cycle": "error",
+    "import/no-duplicates": ["error", { "prefer-inline": true }],
+    "import/no-extraneous-dependencies": ["error"],
+    "import/no-internal-modules": ["error", { "allow": ["*/lib/*"] }],
+    "import/no-named-as-default": "off",
+    "import/no-named-as-default-member": "off",
+    "quotes": ["error", "single", { "avoidEscape": true }]
+  },
+  "overrides": [
+    {
+      "extends": ["plugin:@typescript-eslint/disable-type-checked"],
+      "files": ["**/*.js"]
+    },
+    {
+      "files": ["**/*.md"],
+      "processor": "markdown/markdown"
+    },
+    {
+      "files": ["./**/*.md/*.js"],
+      "rules": {
+        "@typescript-eslint/no-unused-vars": "off"
+      }
+    }
+  ],
+  "settings": {
+    "import/resolver": {
+      "typescript": true
+    }
+  }
+}

.github/workflows/ci.yml

@@ -71,6 +71,28 @@ jobs:
       - name: 'Run formatter'
         run: npx prettier --ignore-unknown --check '**'
 
+  lint:
+    name: 'Lint'
+    runs-on: ubuntu-22.04
+    timeout-minutes: 30
+    if: github.event_name != 'schedule'
+
+    steps:
+      - name: 'Checkout code'
+        uses: actions/[email protected]
+
+      - name: 'Set up Node.js'
+        uses: actions/[email protected]
+        with:
+          node-version-file: package.json
+          cache: ${{ !env.ACT && 'npm' || '' }}
+
+      - name: 'Install dependencies'
+        run: npm ci
+
+      - name: 'Run the linter'
+        run: npx eslint . --max-warnings 0
+
   typecheck:
     name: 'Typecheck'
     runs-on: ubuntu-22.04
@@ -103,6 +125,7 @@ jobs:
     needs:
       - build-image
       - format
+      - lint
       - typecheck
 
     steps: