Add some rules

.eslintrc.js

@@ -28,12 +28,14 @@ module.exports = {
     'import/no-unresolved': 'error',
     'import/named': 'error',
     'prettier/prettier': ['error', { singleQuote: true, trailingComma: 'es5' }],
+    'react/button-has-type': 'error',
     'react/no-access-state-in-setstate': 'error',
     'react/no-danger': 'error',
     'react/no-did-mount-set-state': 'error',
     'react/no-did-update-set-state': 'error',
     'react/no-will-update-set-state': 'error',
     'react/no-redundant-should-component-update': 'error',
+    'react/no-this-in-sfc': 'error',
     'react/no-typos': 'error',
     // `no-unused-prop-types` is buggy when we use destructuring parameters in
     // functions as it misunderstands them as functional components.
@@ -46,6 +48,13 @@ module.exports = {
       { ignorePureComponents: true },
     ],
     'react/jsx-no-bind': 'error',
+    // We still have too many files without the file annotation. Let's enable it
+    // once we flow-type everything.
+    // 'flowtype/require-valid-file-annotation': [ 'error', 'always', { annotationStyle: 'line' } ],
+    // no-dupe-keys crashes with recent eslint. See
+    // https://github.com/gajus/eslint-plugin-flowtype/pull/266 and
+    // https://github.com/gajus/eslint-plugin-flowtype/pull/302
+    // 'flowtype/no-dupe-keys': 'error',
 
     // overriding recommended rules
     'no-constant-condition': ['error', { checkLoops: false }],
@@ -73,7 +82,9 @@ module.exports = {
     'no-self-compare': 'error',
     'no-throw-literal': 'error',
     'no-unmodified-loop-condition': 'error',
-    'no-unused-expressions': 'error',
+    // We use the version from the flowtype plugin so that flow assertions don't
+    // output an error.
+    'flowtype/no-unused-expressions': 'error',
     'no-useless-call': 'error',
     'no-useless-computed-key': 'error',
     'no-useless-concat': 'error',

src/components/app/FooterLinks.js

@@ -30,6 +30,7 @@ class FooterLinks extends PureComponent<{||}, State> {
           aria-label="Hide links to legal information"
           title="Hide links to legal information"
           className="appFooterLinksClose"
+          type="button"
           onClick={this._onClick}
         >
           ✕

src/components/header/SelectionScrubberOverlay.js

@@ -132,6 +132,7 @@ export default class SelectionScrubberOverlay extends PureComponent<Props> {
               className={classNames('selectionScrubberZoomButton', {
                 hidden: isModifying,
               })}
+              type="button"
               onMouseDown={this._zoomButtonOnMouseDown}
               onClick={this._zoomButtonOnClick}
             />

src/test/components/__snapshots__/Home.test.js.snap

@@ -116,6 +116,7 @@ exports[`app/Home renders the information screen for other browsers 1`] = `
       className="appFooterLinksClose"
       onClick={[Function]}
       title="Hide links to legal information"
+      type="button"
     >
       ✕
     </button>
@@ -271,6 +272,7 @@ exports[`app/Home renders the install screen for the extension 1`] = `
       className="appFooterLinksClose"
       onClick={[Function]}
       title="Hide links to legal information"
+      type="button"
     >
       ✕
     </button>
@@ -460,6 +462,7 @@ exports[`app/Home renders the install screen for the legacy add-on 1`] = `
       className="appFooterLinksClose"
       onClick={[Function]}
       title="Hide links to legal information"
+      type="button"
     >
       ✕
     </button>
@@ -633,6 +636,7 @@ exports[`app/Home renders the usage instructions for pages with the extension in
       className="appFooterLinksClose"
       onClick={[Function]}
       title="Hide links to legal information"
+      type="button"
     >
       ✕
     </button>

src/utils/flow.js

@@ -48,7 +48,7 @@ export function toValidTabSlug(tabSlug: any): TabSlug | null {
     default: {
       // The coerced type SHOULD be empty here. If in reality we get
       // here, then it's not a valid transform type, so return null.
-      (coercedTabSlug: empty); // eslint-disable-line no-unused-expressions
+      (coercedTabSlug: empty);
       return null;
     }
   }
@@ -76,7 +76,7 @@ export function convertToTransformType(type: string): TransformType | null {
     default: {
       // The coerced type SHOULD be empty here. If in reality we get
       // here, then it's not a valid transform type, so return null.
-      (coercedType: empty); // eslint-disable-line no-unused-expressions
+      (coercedType: empty);
       return null;
     }
   }