From c59337962804b00e48ed996fd6230f3924d05ad2 Mon Sep 17 00:00:00 2001
From: Simeon Cheeseman <simeon@tablecheck.com>
Date: Thu, 24 Mar 2022 13:38:04 +0900
Subject: [PATCH] [FIX] `function-component-definition`: replace `var` by
 `const` in certain situations

Preserves existing variable definition if it exists.
---
 lib/rules/function-component-definition.js    | 35 ++++++++++++---
 .../rules/function-component-definition.js    | 43 +++++++++++++++----
 2 files changed, 64 insertions(+), 14 deletions(-)

diff --git a/lib/rules/function-component-definition.js b/lib/rules/function-component-definition.js
index f5dd96e16e..f326db3c99 100644
--- a/lib/rules/function-component-definition.js
+++ b/lib/rules/function-component-definition.js
@@ -21,8 +21,8 @@ function buildFunction(template, parts) {
 
 const NAMED_FUNCTION_TEMPLATES = {
   'function-declaration': 'function {name}{typeParams}({params}){returnType} {body}',
-  'arrow-function': 'const {name}{typeAnnotation} = {typeParams}({params}){returnType} => {body}',
-  'function-expression': 'const {name}{typeAnnotation} = function{typeParams}({params}){returnType} {body}',
+  'arrow-function': '{varType} {name}{typeAnnotation} = {typeParams}({params}){returnType} => {body}',
+  'function-expression': '{varType} {name}{typeAnnotation} = function{typeParams}({params}){returnType} {body}',
 };
 
 const UNNAMED_FUNCTION_TEMPLATES = {
@@ -145,6 +145,7 @@ module.exports = {
 
   create: Components.detect((context, components) => {
     const configuration = context.options[0] || {};
+    let fileVarType = 'var';
 
     const namedConfig = [].concat(configuration.namedComponents || 'function-declaration');
     const unnamedConfig = [].concat(configuration.unnamedComponents || 'function-expression');
@@ -159,6 +160,10 @@ module.exports = {
       if (options.type === 'arrow-function' && hasOneUnconstrainedTypeParam(node)) return;
       if (isUnfixableBecauseOfExport(node)) return;
       if (isFunctionExpressionWithName(node)) return;
+      let varType = fileVarType;
+      if ((node.type === 'FunctionExpression' || node.type === 'ArrowFunctionExpression') && node.parent.type === 'VariableDeclarator') {
+        varType = node.parent.parent.kind;
+      }
 
       return (fixer) => fixer.replaceTextRange(options.range, buildFunction(options.template, {
         typeAnnotation,
@@ -167,6 +172,7 @@ module.exports = {
         returnType: getNodeText(node.returnType, source),
         body: getBody(node, source),
         name: getName(node),
+        varType,
       }));
     }
 
@@ -209,11 +215,28 @@ module.exports = {
     // --------------------------------------------------------------------------
     // Public
     // --------------------------------------------------------------------------
-
+    const validatePairs = [];
+    let hasEs6OrJsx = false;
     return {
-      FunctionDeclaration(node) { validate(node, 'function-declaration'); },
-      ArrowFunctionExpression(node) { validate(node, 'arrow-function'); },
-      FunctionExpression(node) { validate(node, 'function-expression'); },
+      FunctionDeclaration(node) {
+        validatePairs.push([node, 'function-declaration']);
+      },
+      ArrowFunctionExpression(node) {
+        validatePairs.push([node, 'arrow-function']);
+      },
+      FunctionExpression(node) {
+        validatePairs.push([node, 'function-expression']);
+      },
+      VariableDeclaration(node) {
+        hasEs6OrJsx = hasEs6OrJsx || node.kind === 'const' || node.kind === 'let';
+      },
+      JSXElement() {
+        hasEs6OrJsx = true;
+      },
+      'Program:exit'() {
+        if (hasEs6OrJsx) fileVarType = 'const';
+        validatePairs.forEach((pair) => validate(pair[0], pair[1]));
+      },
     };
   }),
 };
diff --git a/tests/lib/rules/function-component-definition.js b/tests/lib/rules/function-component-definition.js
index 2da6396a99..dc872bfed2 100644
--- a/tests/lib/rules/function-component-definition.js
+++ b/tests/lib/rules/function-component-definition.js
@@ -57,6 +57,10 @@ ruleTester.run('function-component-definition', rule, {
       code: 'var Hello = (props) => { return <div/> }',
       options: [{ namedComponents: 'arrow-function' }],
     },
+    {
+      code: 'const Hello = (props) => { return <div/> }',
+      options: [{ namedComponents: 'arrow-function' }],
+    },
     {
       code: 'function Hello(props) { return <div/> }',
       options: [{ namedComponents: 'function-declaration' }],
@@ -65,6 +69,10 @@ ruleTester.run('function-component-definition', rule, {
       code: 'var Hello = function(props) { return <div/> }',
       options: [{ namedComponents: 'function-expression' }],
     },
+    {
+      code: 'const Hello = function(props) { return <div/> }',
+      options: [{ namedComponents: 'function-expression' }],
+    },
     {
       code: 'function Hello() { return function() { return <div/> } }',
       options: [{ unnamedComponents: 'function-expression' }],
@@ -77,6 +85,10 @@ ruleTester.run('function-component-definition', rule, {
       code: 'var Foo = React.memo(function Foo() { return <p/> })',
       options: [{ namedComponents: 'function-declaration' }],
     },
+    {
+      code: 'const Foo = React.memo(function Foo() { return <p/> })',
+      options: [{ namedComponents: 'function-declaration' }],
+    },
     {
       // shouldn't trigger this rule since functions stating with a lowercase
       // letter are not considered components
@@ -404,7 +416,7 @@ ruleTester.run('function-component-definition', rule, {
         }
       `,
       output: `
-        var Hello = (props) => {
+        const Hello = (props) => {
           return <div/>;
         }
       `,
@@ -474,7 +486,7 @@ ruleTester.run('function-component-definition', rule, {
         }
       `,
       output: `
-        var Hello = function(props) {
+        const Hello = function(props) {
           return <div/>;
         }
       `,
@@ -554,7 +566,7 @@ ruleTester.run('function-component-definition', rule, {
         }
       `,
       output: `
-        var Hello = (props: Test) => {
+        const Hello = (props: Test) => {
           return <div/>;
         }
       `,
@@ -584,7 +596,7 @@ ruleTester.run('function-component-definition', rule, {
         }
       `,
       output: `
-        var Hello = function(props: Test) {
+        const Hello = function(props: Test) {
           return <div/>;
         }
       `,
@@ -592,6 +604,21 @@ ruleTester.run('function-component-definition', rule, {
       errors: [{ messageId: 'function-expression' }],
       features: ['types'],
     },
+    {
+      code: `
+        function Hello(props: Test) {
+          return React.createElement('div');
+        }
+      `,
+      output: `
+        var Hello = function(props: Test) {
+          return React.createElement('div');
+        }
+      `,
+      options: [{ namedComponents: 'function-expression' }],
+      errors: [{ messageId: 'function-expression' }],
+      features: ['types'],
+    },
     {
       code: `
         var Hello = (props: Test) => {
@@ -674,7 +701,7 @@ ruleTester.run('function-component-definition', rule, {
         }
       `,
       output: `
-        var Hello = <Test extends {}>(props: Test) => {
+        const Hello = <Test extends {}>(props: Test) => {
           return <div/>;
         }
       `,
@@ -704,7 +731,7 @@ ruleTester.run('function-component-definition', rule, {
         }
       `,
       output: `
-        var Hello = function<Test extends {}>(props: Test) {
+        const Hello = function<Test extends {}>(props: Test) {
           return <div/>;
         }
       `,
@@ -874,7 +901,7 @@ ruleTester.run('function-component-definition', rule, {
         }
       `,
       output: `
-        export var Hello = (props) => {
+        export const Hello = (props) => {
           return <div/>;
         }
       `,
@@ -934,7 +961,7 @@ ruleTester.run('function-component-definition', rule, {
         }
       `,
       output: `
-        var Hello = (props) => {
+        const Hello = (props) => {
           return <div/>;
         }
       `,