[Fix] no-unused-state: TS: support getDerivedStateFromProps as an arrow function

Fixes #2061

Author: ljharb

CHANGELOG.md

@@ -21,6 +21,7 @@ This change log adheres to standards from [Keep a CHANGELOG](https://keepachange
 * [`sort-prop-types`]: avoid repeated warnings of the same node/reason ([#519][] @ljharb)
 * [`jsx-indent`]: Fix indent handling for closing parentheses ([#620][] @stefanbuck])
 * [`prop-types`/`propTypes`]: follow a returned identifier to see if it is JSX ([#1046][] @ljharb)
+* [`no-unused-state`]: TS: support `getDerivedStateFromProps` as an arrow function ([#2061][] @ljharb)
 
 ### Changed
 * [readme] change [`jsx-runtime`] link from branch to sha ([#3160][] @tatsushitoji)
@@ -51,6 +52,7 @@ This change log adheres to standards from [Keep a CHANGELOG](https://keepachange
 [#3133]: https://github.com/yannickcr/eslint-plugin-react/pull/3133
 [#2921]: https://github.com/yannickcr/eslint-plugin-react/pull/2921
 [#2753]: https://github.com/yannickcr/eslint-plugin-react/pull/2753
+[#2061]: https://github.com/yannickcr/eslint-plugin-react/issues/2061
 [#1817]: https://github.com/yannickcr/eslint-plugin-react/issues/1817
 [#1815]: https://github.com/yannickcr/eslint-plugin-react/issues/1815
 [#1754]: https://github.com/yannickcr/eslint-plugin-react/issues/1754

lib/rules/no-unused-state.js

@@ -243,6 +243,18 @@ module.exports = {
       classInfo = null;
     }
 
+    function isGDSFP(node) {
+      const name = getName(node.key);
+      if (
+        !node.static
+        || name !== 'getDerivedStateFromProps'
+        || node.value.params.length < 2 // no `state` argument
+      ) {
+        return false;
+      }
+      return true;
+    }
+
     return {
       ClassDeclaration: handleES6ComponentEnter,
 
@@ -307,15 +319,13 @@ module.exports = {
       },
 
       'ClassProperty, PropertyDefinition'(node) {
-        if (!classInfo) {
-          return;
-        }
         // If we see state being assigned as a class property using an object
         // expression, record all the fields of that object as state fields.
         const unwrappedValueNode = ast.unwrapTSAsExpression(node.value);
 
+        const name = getName(node.key);
         if (
-          getName(node.key) === 'state'
+          name === 'state'
           && !node.static
           && unwrappedValueNode
           && unwrappedValueNode.type === 'ObjectExpression'
@@ -345,12 +355,39 @@ module.exports = {
         }
       },
 
+      'PropertyDefinition, ClassProperty'(node) {
+        if (!isGDSFP(node)) {
+          return;
+        }
+
+        const childScope = context.getScope().childScopes.find((x) => x.block === node.value);
+        if (!childScope) {
+          return;
+        }
+        const scope = childScope.variableScope.childScopes.find((x) => x.block === node.value);
+        const stateArg = node.value.params[1]; // probably "state"
+        if (!scope.variables) {
+          return;
+        }
+        const argVar = scope.variables.find((x) => x.name === stateArg.name);
+
+        const stateRefs = argVar.references;
+
+        stateRefs.forEach((ref) => {
+          const identifier = ref.identifier;
+          if (identifier && identifier.parent && identifier.parent.type === 'MemberExpression') {
+            addUsedStateField(identifier.parent.property);
+          }
+        });
+      },
+
       'PropertyDefinition:exit'(node) {
         if (
           classInfo
           && !node.static
           && node.value
           && node.value.type === 'ArrowFunctionExpression'
+          && !isGDSFP(node)
         ) {
           // Forget our set of local aliases.
           classInfo.aliases = null;

tests/lib/rules/no-unused-state.js

@@ -4,7 +4,9 @@
 
 'use strict';
 
+const semver = require('semver');
 const RuleTester = require('eslint').RuleTester;
+const tsEslintVersion = require('@typescript-eslint/parser/package.json').version;
 const rule = require('../../../lib/rules/no-unused-state');
 
 const parsers = require('../../helpers/parsers');
@@ -26,7 +28,7 @@ function getErrorMessages(unusedFields) {
 }
 
 eslintTester.run('no-unused-state', rule, {
-  valid: parsers.all([
+  valid: parsers.all([].concat(
     {
       code: `
         function StatelessFnUnaffectedTest(props) {
@@ -984,7 +986,31 @@ eslintTester.run('no-unused-state', rule, {
       `,
       features: ['ts', 'no-babel'],
     },
-  ]),
+    semver.satisfies(tsEslintVersion, '>= 5') ? {
+      code: `
+        interface Props {}
+        
+        interface State {
+          flag: boolean;
+        }
+        
+        export default class RuleTest extends React.Component<Props, State> {
+          readonly state: State = {
+            flag: false,
+          };
+        
+          static getDerivedStateFromProps = (props: Props, state: State) => {
+            const newState: Partial<State> = {};
+            if (!state.flag) {
+              newState.flag = true;
+            }
+            return newState;
+          };
+        }
+      `,
+      features: ['ts', 'no-babel-old', 'no-ts-old'],
+    } : []
+  )),
 
   invalid: parsers.all([
     {