feat: add allowTypeAnnotation to func-style (#19754)

sethamus · web-flow · commit c4a6b6051889 · 2025-05-28T23:35:23.000+05:30
diff --git a/docs/src/rules/func-style.md b/docs/src/rules/func-style.md
@@ -64,6 +64,7 @@ This rule has a string option:
 This rule has an object option for two exceptions:
 
 * `"allowArrowFunctions"`: `true` (default `false`) allows the use of arrow functions. This option applies only when the string option is set to `"declaration"` (arrow functions are always allowed when the string option is set to `"expression"`, regardless of this option)
+* `"allowTypeAnnotation"`: `true` (default `false`) allows the use of function expressions and arrow functions when the variable declaration has type annotation, regardless of the `allowArrowFunctions` option. This option applies only when the string option is set to `"declaration"`. (TypeScript only)
 * `"overrides"`:
     * `"namedExports": "expression" | "declaration" | "ignore"`: used to override function styles in named exports
         * `"expression"`: like string option
@@ -173,6 +174,24 @@ const foo = () => {};
 
 :::
 
+### allowTypeAnnotation
+
+Examples of **correct** TypeScript code for this rule with the `"declaration", { "allowTypeAnnotation": true }` options:
+
+::: correct
+
+```ts
+/*eslint func-style: ["error", "declaration", { "allowTypeAnnotation": true }]*/
+
+type Fn = () => undefined;
+
+const foo: Fn = function() {};
+
+const bar: Fn = () => {};
+```
+
+:::
+
 ### overrides
 
 #### namedExports
diff --git a/lib/rules/func-style.js b/lib/rules/func-style.js
@@ -11,12 +11,15 @@
 /** @type {import('../types').Rule.RuleModule} */
 module.exports = {
 	meta: {
+		dialects: ["javascript", "typescript"],
+		language: "javascript",
 		type: "suggestion",
 
 		defaultOptions: [
 			"expression",
 			{
 				allowArrowFunctions: false,
+				allowTypeAnnotation: false,
 				overrides: {},
 			},
 		],
@@ -39,6 +42,9 @@ module.exports = {
 					allowArrowFunctions: {
 						type: "boolean",
 					},
+					allowTypeAnnotation: {
+						type: "boolean",
+					},
 					overrides: {
 						type: "object",
 						properties: {
@@ -60,7 +66,8 @@ module.exports = {
 	},
 
 	create(context) {
-		const [style, { allowArrowFunctions, overrides }] = context.options;
+		const [style, { allowArrowFunctions, allowTypeAnnotation, overrides }] =
+			context.options;
 		const enforceDeclarations = style === "declaration";
 		const { namedExports: exportFunctionStyle } = overrides;
 		const stack = [];
@@ -136,7 +143,8 @@ module.exports = {
 					node.parent.type === "VariableDeclarator" &&
 					(typeof exportFunctionStyle === "undefined" ||
 						node.parent.parent.parent.type !==
-							"ExportNamedDeclaration")
+							"ExportNamedDeclaration") &&
+					!(allowTypeAnnotation && node.parent.id.typeAnnotation)
 				) {
 					context.report({
 						node: node.parent,
@@ -148,7 +156,8 @@ module.exports = {
 					node.parent.type === "VariableDeclarator" &&
 					node.parent.parent.parent.type ===
 						"ExportNamedDeclaration" &&
-					exportFunctionStyle === "declaration"
+					exportFunctionStyle === "declaration" &&
+					!(allowTypeAnnotation && node.parent.id.typeAnnotation)
 				) {
 					context.report({
 						node: node.parent,
@@ -183,7 +192,8 @@ module.exports = {
 						enforceDeclarations &&
 						(typeof exportFunctionStyle === "undefined" ||
 							node.parent.parent.parent.type !==
-								"ExportNamedDeclaration")
+								"ExportNamedDeclaration") &&
+						!(allowTypeAnnotation && node.parent.id.typeAnnotation)
 					) {
 						context.report({
 							node: node.parent,
@@ -194,7 +204,8 @@ module.exports = {
 					if (
 						node.parent.parent.parent.type ===
 							"ExportNamedDeclaration" &&
-						exportFunctionStyle === "declaration"
+						exportFunctionStyle === "declaration" &&
+						!(allowTypeAnnotation && node.parent.id.typeAnnotation)
 					) {
 						context.report({
 							node: node.parent,
diff --git a/lib/types/rules.d.ts b/lib/types/rules.d.ts
@@ -807,6 +807,10 @@ export interface ESLintRules extends Linter.RulesRecord {
 				 * @default false
 				 */
 				allowArrowFunctions: boolean;
+				/**
+				 * @default false
+				 */
+				allowTypeAnnotation: boolean;
 				overrides: {
 					namedExports: "declaration" | "expression" | "ignore";
 				};
diff --git a/tests/lib/rules/func-style.js b/tests/lib/rules/func-style.js
@@ -422,6 +422,16 @@ ruleTester.run("func-style", rule, {
 				},
 			],
 		},
+		{
+			code: "const foo = function() {};",
+			options: ["declaration", { allowTypeAnnotation: true }],
+			errors: [
+				{
+					messageId: "declaration",
+					type: "VariableDeclarator",
+				},
+			],
+		},
 		{
 			code: "$1: function $2() { }",
 			languageOptions: { sourceType: "script" },
@@ -432,6 +442,48 @@ ruleTester.run("func-style", rule, {
 				},
 			],
 		},
+		{
+			code: "const foo = () => {};",
+			options: ["declaration", { allowTypeAnnotation: true }],
+			errors: [
+				{
+					messageId: "declaration",
+					type: "VariableDeclarator",
+				},
+			],
+		},
+		{
+			code: "export const foo = function() {};",
+			options: [
+				"expression",
+				{
+					allowTypeAnnotation: true,
+					overrides: { namedExports: "declaration" },
+				},
+			],
+			errors: [
+				{
+					messageId: "declaration",
+					type: "VariableDeclarator",
+				},
+			],
+		},
+		{
+			code: "export const foo = () => {};",
+			options: [
+				"expression",
+				{
+					allowTypeAnnotation: true,
+					overrides: { namedExports: "declaration" },
+				},
+			],
+			errors: [
+				{
+					messageId: "declaration",
+					type: "VariableDeclarator",
+				},
+			],
+		},
 		{
 			code: "if (foo) function bar() {}",
 			languageOptions: { sourceType: "script" },
@@ -561,6 +613,42 @@ ruleTesterTypeScript.run("func-style", rule, {
 			code: "export const foo: () => void = function(): void {};",
 			options: ["expression", { overrides: { namedExports: "ignore" } }],
 		},
+		{
+			code: "const expression: Fn = function () {}",
+			options: ["declaration", { allowTypeAnnotation: true }],
+		},
+		{
+			code: "const arrow: Fn = () => {}",
+			options: ["declaration", { allowTypeAnnotation: true }],
+		},
+		{
+			code: "export const expression: Fn = function () {}",
+			options: ["declaration", { allowTypeAnnotation: true }],
+		},
+		{
+			code: "export const arrow: Fn = () => {}",
+			options: ["declaration", { allowTypeAnnotation: true }],
+		},
+		{
+			code: "export const expression: Fn = function () {}",
+			options: [
+				"expression",
+				{
+					allowTypeAnnotation: true,
+					overrides: { namedExports: "declaration" },
+				},
+			],
+		},
+		{
+			code: "export const arrow: Fn = () => {}",
+			options: [
+				"expression",
+				{
+					allowTypeAnnotation: true,
+					overrides: { namedExports: "declaration" },
+				},
+			],
+		},
 		{
 			code: "$1: function $2(): void { }",
 			options: ["declaration"],
