[Fix] display-name: fix false positive for assignment of function returning null

Author: apbarrero

lib/util/Components.js

@@ -508,6 +508,15 @@ function componentRule(rule, context) {
           return undefined;
         }
 
+        // case: any = () => { return => null }
+        // case: any = () => null
+        if (node.parent.type === 'AssignmentExpression' && !isPropertyAssignment && utils.isReturningJSXOrNull(node)) {
+          if (isFirstLetterCapitalized(node.parent.left.name)) {
+            return node;
+          }
+          return undefined;
+        }
+
         // for case abc = { [someobject.somekey]: props => { ... return not-jsx } }
         if (node.parent && node.parent.key && node.parent.key.type === 'MemberExpression' && !utils.isReturningJSX(node) && !utils.isReturningOnlyNull(node)) {
           return undefined;

tests/lib/rules/display-name.js

@@ -588,6 +588,15 @@ ruleTester.run('display-name', rule, {
         }
       `,
     },
+    {
+      // issue #3329
+      code: `
+        let demo = null;
+        demo = (a) => {
+          if (a == null) return null;
+          return f(a);
+        };`,
+    },
     {
       // issue #3303
       code: `