Add prevState and nextState support to no-unused-state (fixes #1759)

lib/rules/no-unused-state.js

@@ -77,7 +77,7 @@ module.exports = {
     // JSX attributes), then this is again set to null.
     let classInfo = null;
 
-    // Returns true if the given node is possibly a reference to `this.state`.
+    // Returns true if the given node is possibly a reference to `this.state`, `prevState` or `nextState`.
     function isStateReference(node) {
       node = uncast(node);
 
@@ -91,7 +91,15 @@ module.exports = {
         classInfo.aliases &&
         classInfo.aliases.has(node.name);
 
-      return isDirectStateReference || isAliasedStateReference;
+      const isPrevStateReference =
+        node.type === 'Identifier' &&
+        node.name === 'prevState';
+
+      const isNextStateReference =
+        node.type === 'Identifier' &&
+        node.name === 'nextState';
+
+      return isDirectStateReference || isAliasedStateReference || isPrevStateReference || isNextStateReference;
     }
 
     // Takes an ObjectExpression node and adds all named Property nodes to the

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

@@ -491,6 +491,49 @@ eslintTester.run('no-unused-state', rule, {
         }
       }`,
       parser: 'babel-eslint'
+    },
+    {
+      code: `class ESLintExample extends Component {
+        constructor(props) {
+          super(props);
+          this.state = {
+            id: 123,
+          };
+        }
+        static getDerivedStateFromProps(nextProps, prevState) {
+          if (prevState.id === nextProps.id) {
+            return {
+              selected: true,
+            };
+          }
+          return null;
+        }
+        render() {
+          return (
+            <h1>{this.state.selected ? 'Selected' : 'Not selected'}</h1>
+          );
+        }
+      }`,
+      parser: 'babel-eslint'
+    },
+    {
+      code: `class ESLintExample extends Component {
+        constructor(props) {
+          super(props);
+          this.state = {
+            id: 123,
+          };
+        }
+        shouldComponentUpdate(nextProps, nextState) {
+          return nextState.id === nextProps.id;
+        }
+        render() {
+          return (
+            <h1>{this.state.selected ? 'Selected' : 'Not selected'}</h1>
+          );
+        }
+      }`,
+      parser: 'babel-eslint'
     }
   ],