fix(eslint-plugin): [explicit-function-return-type] support AllowTypedFunctionExpression within AllowHigherOrderFunction

packages/eslint-plugin/src/rules/explicit-function-return-type.ts

@@ -5,7 +5,8 @@ import {
 import * as util from '../util';
 import {
   checkFunctionReturnType,
-  checkFunctionExpressionReturnType,
+  isValidFunctionExpressionReturnType,
+  ancestorHasReturnType,
 } from '../util/explicitReturnTypeUtils';
 
 type Options = [
@@ -81,7 +82,15 @@ export default util.createRule<Options, MessageIds>({
           return;
         }
 
-        checkFunctionExpressionReturnType(node, options, sourceCode, loc =>
+        if (
+          options.allowTypedFunctionExpressions &&
+          (isValidFunctionExpressionReturnType(node, options) ||
+            ancestorHasReturnType(node))
+        ) {
+          return;
+        }
+
+        checkFunctionReturnType(node, options, sourceCode, loc =>
           context.report({
             node,
             loc,
@@ -90,6 +99,10 @@ export default util.createRule<Options, MessageIds>({
         );
       },
       FunctionDeclaration(node): void {
+        if (options.allowTypedFunctionExpressions && node.returnType) {
+          return;
+        }
+
         checkFunctionReturnType(node, options, sourceCode, loc =>
           context.report({
             node,

packages/eslint-plugin/src/rules/explicit-module-boundary-types.ts

@@ -11,6 +11,7 @@ import {
   FunctionExpression,
   FunctionNode,
   isTypedFunctionExpression,
+  ancestorHasReturnType,
 } from '../util/explicitReturnTypeUtils';
 
 type Options = [
@@ -390,55 +391,6 @@ export default util.createRule<Options, MessageIds>({
       }
     }
 
-    /**
-     * Check whether any ancestor of the provided function has a valid return type.
-     * This function assumes that the function either:
-     * - belongs to an exported function chain validated by isExportedHigherOrderFunction
-     * - is directly exported itself
-     */
-    function ancestorHasReturnType(node: FunctionNode): boolean {
-      let ancestor = node.parent;
-
-      if (ancestor?.type === AST_NODE_TYPES.Property) {
-        ancestor = ancestor.value;
-      }
-
-      // if the ancestor is not a return, then this function was not returned at all, so we can exit early
-      const isReturnStatement =
-        ancestor?.type === AST_NODE_TYPES.ReturnStatement;
-      const isBodylessArrow =
-        ancestor?.type === AST_NODE_TYPES.ArrowFunctionExpression &&
-        ancestor.body.type !== AST_NODE_TYPES.BlockStatement;
-      if (!isReturnStatement && !isBodylessArrow) {
-        return false;
-      }
-
-      while (ancestor) {
-        switch (ancestor.type) {
-          case AST_NODE_TYPES.ArrowFunctionExpression:
-          case AST_NODE_TYPES.FunctionExpression:
-          case AST_NODE_TYPES.FunctionDeclaration:
-            if (ancestor.returnType) {
-              return true;
-            }
-            // assume
-            break;
-
-          // const x: Foo = () => {};
-          // Assume that a typed variable types the function expression
-          case AST_NODE_TYPES.VariableDeclarator:
-            if (ancestor.id.typeAnnotation) {
-              return true;
-            }
-            break;
-        }
-
-        ancestor = ancestor.parent;
-      }
-
-      return false;
-    }
-
     function checkEmptyBodyFunctionExpression(
       node: TSESTree.TSEmptyBodyFunctionExpression,
     ): void {

packages/eslint-plugin/src/util/explicitReturnTypeUtils.ts

@@ -287,11 +287,57 @@ function checkFunctionExpressionReturnType(
   checkFunctionReturnType(node, options, sourceCode, report);
 }
 
+/**
+ * Check whether any ancestor of the provided function has a valid return type.
+ */
+function ancestorHasReturnType(node: FunctionNode): boolean {
+  let ancestor = node.parent;
+
+  if (ancestor?.type === AST_NODE_TYPES.Property) {
+    ancestor = ancestor.value;
+  }
+
+  // if the ancestor is not a return, then this function was not returned at all, so we can exit early
+  const isReturnStatement = ancestor?.type === AST_NODE_TYPES.ReturnStatement;
+  const isBodylessArrow =
+    ancestor?.type === AST_NODE_TYPES.ArrowFunctionExpression &&
+    ancestor.body.type !== AST_NODE_TYPES.BlockStatement;
+  if (!isReturnStatement && !isBodylessArrow) {
+    return false;
+  }
+
+  while (ancestor) {
+    switch (ancestor.type) {
+      case AST_NODE_TYPES.ArrowFunctionExpression:
+      case AST_NODE_TYPES.FunctionExpression:
+      case AST_NODE_TYPES.FunctionDeclaration:
+        if (ancestor.returnType) {
+          return true;
+        }
+        break;
+
+      // const x: Foo = () => {};
+      // Assume that a typed variable types the function expression
+      case AST_NODE_TYPES.VariableDeclarator:
+        if (ancestor.id.typeAnnotation) {
+          return true;
+        }
+        break;
+    }
+
+    ancestor = ancestor.parent;
+  }
+
+  return false;
+}
+
 export {
   checkFunctionExpressionReturnType,
   checkFunctionReturnType,
   doesImmediatelyReturnFunctionExpression,
   FunctionExpression,
   FunctionNode,
   isTypedFunctionExpression,
+  isValidFunctionExpressionReturnType,
+  ancestorHasReturnType,
 };

packages/eslint-plugin/tests/rules/explicit-function-return-type.test.ts

@@ -400,6 +400,68 @@ new Foo(1, () => {});
       code: 'const log = (message: string) => void console.log(message);',
       options: [{ allowConciseArrowFunctionExpressionsStartingWithVoid: true }],
     },
+    {
+      filename: 'test.ts',
+      code: `
+type HigherOrderType = () => (arg1: string) => (arg2: number) => string;
+const x: HigherOrderType = () => arg1 => arg2 => 'foo';
+      `,
+      options: [
+        {
+          allowTypedFunctionExpressions: true,
+          allowHigherOrderFunctions: true,
+        },
+      ],
+    },
+    {
+      filename: 'test.ts',
+      code: `
+type HigherOrderType = () => (arg1: string) => (arg2: number) => string;
+const x: HigherOrderType = () => arg1 => arg2 => 'foo';
+      `,
+      options: [
+        {
+          allowTypedFunctionExpressions: true,
+          allowHigherOrderFunctions: false,
+        },
+      ],
+    },
+    {
+      filename: 'test.ts',
+      code: `
+interface Foo {
+  foo: string;
+  arrowFn: () => string;
+}
+
+function foo(): Foo {
+  return {
+    foo: 'foo',
+    arrowFn: () => 'test',
+  };
+}
+      `,
+      options: [
+        {
+          allowTypedFunctionExpressions: true,
+          allowHigherOrderFunctions: true,
+        },
+      ],
+    },
+    {
+      filename: 'test.ts',
+      code: `
+type Foo = (arg1: string) => string;
+type Bar<T> = (arg2: string) => T;
+const x: Bar<Foo> = arg1 => arg2 => arg1 + arg2;
+      `,
+      options: [
+        {
+          allowTypedFunctionExpressions: true,
+          allowHigherOrderFunctions: true,
+        },
+      ],
+    },
   ],
   invalid: [
     {
@@ -1027,6 +1089,64 @@ foo({
     {
       filename: 'test.ts',
       code: `
+type HigherOrderType = () => (arg1: string) => (arg2: number) => string;
+const x: HigherOrderType = () => arg1 => arg2 => 'foo';
+      `,
+      options: [
+        {
+          allowTypedFunctionExpressions: false,
+          allowHigherOrderFunctions: true,
+        },
+      ],
+      errors: [
+        {
+          messageId: 'missingReturnType',
+          line: 3,
+          endLine: 3,
+          column: 42,
+          endColumn: 49,
+        },
+      ],
+    },
+    {
+      filename: 'test.ts',
+      code: `
+type HigherOrderType = () => (arg1: string) => (arg2: number) => string;
+const x: HigherOrderType = () => arg1 => arg2 => 'foo';
+      `,
+      options: [
+        {
+          allowTypedFunctionExpressions: false,
+          allowHigherOrderFunctions: false,
+        },
+      ],
+      errors: [
+        {
+          messageId: 'missingReturnType',
+          line: 3,
+          endLine: 3,
+          column: 28,
+          endColumn: 33,
+        },
+        {
+          messageId: 'missingReturnType',
+          line: 3,
+          endLine: 3,
+          column: 34,
+          endColumn: 41,
+        },
+        {
+          messageId: 'missingReturnType',
+          line: 3,
+          endLine: 3,
+          column: 42,
+          endColumn: 49,
+        },
+      ],
+    },
+    {
+      filename: 'test.ts',
+      code: `
 const func = (value: number) => ({ type: 'X', value } as any);
 const func = (value: number) => ({ type: 'X', value } as Action);
       `,