[eslint-plugin-react-hooks] update naming rules to disallow _component

Author: kassens

packages/eslint-plugin-react-hooks/CHANGELOG.md

@@ -1,3 +1,10 @@
+## Next Release
+
+* **New Violations:** Components now need to be named with a capital letter, potentially prefixed with an underscore. E.g. `Button` or `_Button`. `_component` is no longer valid. ([@kassens](https://github.com/kassens)) in [#25162](https://github.com/facebook/react/pull/25162)
+* Hooks can now have an optional underscore prefix, e.g. `_useName`. ([@kassens](https://github.com/kassens)) in [#25162](https://github.com/facebook/react/pull/25162)
+
+## 4.6.0
+
 ## 4.5.0
 
 * Fix false positive error with large number of branches. ([@scyron6](https://github.com/scyron6) in [#24287](https://github.com/facebook/react/pull/24287))

packages/eslint-plugin-react-hooks/__tests__/ESLintRulesOfHooks-test.js

@@ -200,11 +200,10 @@ const tests = {
       })
     `,
     `
-      // Valid because they're not matching use[A-Z].
+      // Valid because they're not matching ^_*use[A-Z].
       fooState();
       use();
       _use();
-      _useState();
       use_hook();
       // also valid because it's not matching the PascalCase namespace
       jest.useFakeTimer()
@@ -406,17 +405,6 @@ const tests = {
         const [myState, setMyState] = useState(null);
       }
     `,
-    `
-      // Valid, but should be invalid. '_useHook' is currently recognized as a component.
-      function Component(props) {
-        if (cond) {
-          _useHook();
-        }
-      }
-      function _useHook() {
-        useState(null);
-      }
-    `,
   ],
   invalid: [
     {
@@ -443,6 +431,7 @@ const tests = {
       `,
       errors: [
         topLevelError('Hook.useState'),
+        topLevelError('Hook._useState'),
         topLevelError('Hook.use42'),
         topLevelError('Hook.useHook'),
       ],
@@ -652,6 +641,20 @@ const tests = {
         functionError('useHookInsideNormalFunction', 'normalFunctionWithHook'),
       ],
     },
+    {
+      code: `
+        // These are neither functions nor hooks.
+        function _normalFunctionWithHook() {
+          useHookInsideNormalFunction();
+        }
+        function _useNotAHook() {
+          useHookInsideNormalFunction();
+        }
+      `,
+      errors: [
+        functionError('useHookInsideNormalFunction', '_normalFunctionWithHook'),
+      ],
+    },
     {
       code: `
         // Invalid because it's dangerous and might not warn otherwise.
@@ -893,17 +896,33 @@ const tests = {
         // they are confusing anyway due to the use*() convention
         // already being associated with Hooks.
         useState();
+        _useState();
         if (foo) {
           const foo = React.useCallback(() => {});
         }
         useCustomHook();
       `,
       errors: [
         topLevelError('useState'),
+        topLevelError('_useState'),
         topLevelError('React.useCallback'),
         topLevelError('useCustomHook'),
       ],
     },
+    {
+      code: `
+        // Valid, but should be invalid. '_useHook' is currently recognized as a component.
+        function Component(props) {
+          if (cond) {
+            _useHook();
+          }
+        }
+        function _useHook() {
+          useState(null);
+        }
+      `,
+      errors: [conditionalError('_useHook')],
+    },
     {
       code: `
         // Technically this is a false positive.

packages/eslint-plugin-react-hooks/src/RulesOfHooks.js

@@ -16,7 +16,7 @@
  */
 
 function isHookName(s) {
-  return /^use[A-Z0-9].*$/.test(s);
+  return /^_*use[A-Z0-9]/.test(s);
 }
 
 /**
@@ -42,16 +42,13 @@ function isHook(node) {
 
 /**
  * Checks if the node is a React component name. React component names must
- * always start with a non-lowercase letter. So `MyComponent` or `_MyComponent`
- * are valid component names for instance.
+ * always start with an uppercase letter, potentially prefixed with `_`.
+ * So `MyComponent` or `_MyComponent` are valid component names for instance.
+ * `_myComponent` is not a valid component name.
  */
 
 function isComponentName(node) {
-  if (node.type === 'Identifier') {
-    return !/^[a-z]/.test(node.name);
-  } else {
-    return false;
-  }
+  return node.type === 'Identifier' && /^_*[A-Z]/.test(node.name);
 }
 
 function isReactFunction(node, functionName) {