From 5df5e4a9976964fcf4dc67e241d4e22ec1370fe0 Mon Sep 17 00:00:00 2001
From: Nitin Kumar <snitin315@gmail.com>
Date: Sat, 24 Apr 2021 02:54:57 +0530
Subject: [PATCH] Update: highlight last write reference for no-unused-vars
 (fixes #14324) (#14335)

* Fix: highlight last write reference for no-unused-vars (fixes #14324)

* test: updates

* Chore: add test case

* Fix: apply suggestions

* Chore: update tests

* Chore: more tests
---
 lib/rules/no-unused-vars.js       | 14 +++++++++---
 tests/lib/rules/no-unused-vars.js | 37 ++++++++++++++++++++++++++-----
 2 files changed, 42 insertions(+), 9 deletions(-)

diff --git a/lib/rules/no-unused-vars.js b/lib/rules/no-unused-vars.js
index 32589099cf4..7619be331fa 100644
--- a/lib/rules/no-unused-vars.js
+++ b/lib/rules/no-unused-vars.js
@@ -624,10 +624,18 @@ module.exports = {
 
                     // Report the first declaration.
                     if (unusedVar.defs.length > 0) {
+
+                        // report last write reference, https://github.com/eslint/eslint/issues/14324
+                        const writeReferences = unusedVar.references.filter(ref => ref.isWrite() && ref.from.variableScope === unusedVar.scope.variableScope);
+
+                        let referenceToReport;
+
+                        if (writeReferences.length > 0) {
+                            referenceToReport = writeReferences[writeReferences.length - 1];
+                        }
+
                         context.report({
-                            node: unusedVar.references.length ? unusedVar.references[
-                                unusedVar.references.length - 1
-                            ].identifier : unusedVar.identifiers[0],
+                            node: referenceToReport ? referenceToReport.identifier : unusedVar.identifiers[0],
                             messageId: "unusedVar",
                             data: unusedVar.references.some(ref => ref.isWrite())
                                 ? getAssignedMessageData(unusedVar)
diff --git a/tests/lib/rules/no-unused-vars.js b/tests/lib/rules/no-unused-vars.js
index db041d46b89..423afa167b8 100644
--- a/tests/lib/rules/no-unused-vars.js
+++ b/tests/lib/rules/no-unused-vars.js
@@ -1062,7 +1062,7 @@ ruleTester.run("no-unused-vars", rule, {
             code: `let myArray = [1,2,3,4].filter((x) => x == 0);
     myArray = myArray.filter((x) => x == 1);`,
             parserOptions: { ecmaVersion: 2015 },
-            errors: [{ ...assignedError("myArray"), line: 2, column: 15 }]
+            errors: [{ ...assignedError("myArray"), line: 2, column: 5 }]
         },
         {
             code: "const a = 1; a += 1;",
@@ -1071,21 +1071,28 @@ ruleTester.run("no-unused-vars", rule, {
         },
         {
             code: "var a = function() { a(); };",
-            errors: [{ ...assignedError("a"), line: 1, column: 22 }]
+            errors: [{ ...assignedError("a"), line: 1, column: 5 }]
         },
         {
             code: "var a = function(){ return function() { a(); } };",
-            errors: [{ ...assignedError("a"), line: 1, column: 41 }]
+            errors: [{ ...assignedError("a"), line: 1, column: 5 }]
         },
         {
             code: "const a = () => { a(); };",
             parserOptions: { ecmaVersion: 2015 },
-            errors: [{ ...assignedError("a"), line: 1, column: 19 }]
+            errors: [{ ...assignedError("a"), line: 1, column: 7 }]
         },
         {
             code: "const a = () => () => { a(); };",
             parserOptions: { ecmaVersion: 2015 },
-            errors: [{ ...assignedError("a"), line: 1, column: 25 }]
+            errors: [{ ...assignedError("a"), line: 1, column: 7 }]
+        },
+
+        // https://github.com/eslint/eslint/issues/14324
+        {
+            code: "let x = [];\nx = x.concat(x);",
+            parserOptions: { ecmaVersion: 2015 },
+            errors: [{ ...assignedError("x"), line: 2, column: 1 }]
         },
         {
 
@@ -1098,7 +1105,25 @@ ruleTester.run("no-unused-vars", rule, {
                 }
             }`,
             parserOptions: { ecmaVersion: 2020 },
-            errors: [{ ...definedError("foo"), line: 3, column: 22 }, { ...assignedError("a"), line: 6, column: 21 }]
+            errors: [{ ...assignedError("a"), line: 2, column: 13 }, { ...definedError("foo"), line: 3, column: 22 }]
+        },
+        {
+            code: `let foo;
+            init();
+            foo = foo + 2;
+            function init() {
+                foo = 1;
+            }`,
+            parserOptions: { ecmaVersion: 2020 },
+            errors: [{ ...assignedError("foo"), line: 3, column: 13 }]
+        },
+        {
+            code: `function foo(n) {
+                if (n < 2) return 1;
+                return n * foo(n - 1);
+            }`,
+            parserOptions: { ecmaVersion: 2020 },
+            errors: [{ ...definedError("foo"), line: 1, column: 10 }]
         },
         {
             code: `let c = 'c'