Lint JavaScript with eslint/prettier

This uses `@thoughtbot/eslint-config/base`, along with the `prettier`
plugin and configuration for formatting and fixing.

One rule is overidden, as using modules is nice in the tests, but this
avoids changing the existing `index.js`.

.eslintrc.json

@@ -0,0 +1,7 @@
+{
+  "extends": ["@thoughtbot/eslint-config/base"],
+  "plugins": ["prettier"],
+  "rules": {
+    "import/extensions": "off"
+  }
+}

.github/workflows/tests.yml

@@ -23,3 +23,11 @@ jobs:
       - uses: actions/setup-node@v4
       - run: npm ci
       - run: npm run lint:css
+
+  lint-js:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v3
+      - uses: actions/setup-node@v4
+      - run: npm ci
+      - run: npm run lint:js

.prettierrc

@@ -0,0 +1,3 @@
+{
+  "singleQuote": true
+}

index.js

@@ -1,69 +1,69 @@
 module.exports = {
-  "extends": ["stylelint-config-standard-scss", "stylelint-config-recommended"],
-  "plugins": [
-    "stylelint-declaration-block-no-ignored-properties",
-    "stylelint-order",
+  extends: ['stylelint-config-standard-scss', 'stylelint-config-recommended'],
+  plugins: [
+    'stylelint-declaration-block-no-ignored-properties',
+    'stylelint-order',
   ],
-  "rules": {
-    "at-rule-no-unknown": null,
-    "color-hex-length": "long",
-    "color-named": "never",
-    "comment-whitespace-inside": "always",
-    "declaration-block-no-redundant-longhand-properties": true,
-    "declaration-no-important": true,
-    "declaration-property-unit-allowed-list": {
-      "line-height": []
+  rules: {
+    'at-rule-no-unknown': null,
+    'color-hex-length': 'long',
+    'color-named': 'never',
+    'comment-whitespace-inside': 'always',
+    'declaration-block-no-redundant-longhand-properties': true,
+    'declaration-no-important': true,
+    'declaration-property-unit-allowed-list': {
+      'line-height': [],
     },
-    "declaration-property-value-disallowed-list": {
-      "/^transition/": ["/all/"]
+    'declaration-property-value-disallowed-list': {
+      '/^transition/': ['/all/'],
     },
-    "function-url-quotes": "always",
-    "length-zero-no-unit": true,
-    "max-nesting-depth": 3,
-    "no-empty-source": true,
-    "order/properties-alphabetical-order": true,
-    "plugin/declaration-block-no-ignored-properties": true,
-    "property-no-unknown": [
+    'function-url-quotes': 'always',
+    'length-zero-no-unit': true,
+    'max-nesting-depth': 3,
+    'no-empty-source': true,
+    'order/properties-alphabetical-order': true,
+    'plugin/declaration-block-no-ignored-properties': true,
+    'property-no-unknown': [
       true,
       {
-        "ignoreProperties": [
-          "text-size"
-        ]
-      }
+        ignoreProperties: [
+          'text-size',
+        ],
+      },
     ],
-    "property-no-vendor-prefix": true,
-    "rule-empty-line-before": [
-      "always",
+    'property-no-vendor-prefix': true,
+    'rule-empty-line-before': [
+      'always',
       {
-        "except": [
-          "first-nested"
+        except: [
+          'first-nested',
+        ],
+        ignore: [
+          'after-comment',
         ],
-        "ignore": [
-          "after-comment"
-        ]
-      }
+      },
     ],
-    "scss/at-else-if-parentheses-space-before": "never",
-    "scss/at-extend-no-missing-placeholder": true,
-    "scss/at-function-parentheses-space-before": "never",
-    "scss/load-no-partial-leading-underscore": true,
-    "scss/at-import-partial-extension-blacklist": ["scss"],
-    "scss/at-mixin-parentheses-space-before": "never",
-    "scss/at-rule-no-unknown": true,
-    "scss/dollar-variable-colon-space-after": "always",
-    "scss/dollar-variable-colon-space-before": "never",
-    "scss/map-keys-quotes": "always",
-    "scss/no-duplicate-dollar-variables": true,
-    "scss/operator-no-unspaced": true,
-    "scss/selector-no-redundant-nesting-selector": true,
-    "scss/selector-no-union-class-name": true,
-    "selector-max-id": 0,
-    "selector-no-qualifying-type": true,
-    "selector-no-vendor-prefix": true,
-    "selector-pseudo-element-colon-notation": "double",
-    "selector-type-case": "lower",
-    "shorthand-property-no-redundant-values": true,
-    "value-keyword-case": "lower",
-    "value-no-vendor-prefix": true
-  }
-}
+    'scss/at-else-if-parentheses-space-before': 'never',
+    'scss/at-extend-no-missing-placeholder': true,
+    'scss/at-function-parentheses-space-before': 'never',
+    'scss/load-no-partial-leading-underscore': true,
+    'scss/at-import-partial-extension-blacklist': ['scss'],
+    'scss/at-mixin-parentheses-space-before': 'never',
+    'scss/at-rule-no-unknown': true,
+    'scss/dollar-variable-colon-space-after': 'always',
+    'scss/dollar-variable-colon-space-before': 'never',
+    'scss/map-keys-quotes': 'always',
+    'scss/no-duplicate-dollar-variables': true,
+    'scss/operator-no-unspaced': true,
+    'scss/selector-no-redundant-nesting-selector': true,
+    'scss/selector-no-union-class-name': true,
+    'selector-max-id': 0,
+    'selector-no-qualifying-type': true,
+    'selector-no-vendor-prefix': true,
+    'selector-pseudo-element-colon-notation': 'double',
+    'selector-type-case': 'lower',
+    'shorthand-property-no-redundant-values': true,
+    'value-keyword-case': 'lower',
+    'value-no-vendor-prefix': true,
+  },
+};