feat(useExplicitFunctionReturnType): support typed function expressions

crates/biome_js_analyze/src/lint/nursery/use_explicit_function_return_type.rs

@@ -4,14 +4,16 @@ use biome_analyze::{
 use biome_console::markup;
 use biome_js_semantic::HasClosureAstNode;
 use biome_js_syntax::{
-    AnyJsBinding, AnyJsExpression, AnyJsFunctionBody, AnyJsStatement, AnyTsType, JsFileSource,
-    JsStatementList, JsSyntaxKind,
+    AnyJsBinding, AnyJsExpression, AnyJsFunctionBody, AnyJsStatement, AnyTsType, JsCallExpression,
+    JsFileSource, JsFormalParameter, JsInitializerClause, JsLanguage, JsObjectExpression,
+    JsParenthesizedExpression, JsPropertyClassMember, JsPropertyObjectMember, JsStatementList,
+    JsSyntaxKind, JsVariableDeclarator,
 };
 use biome_js_syntax::{
     AnyJsFunction, JsGetterClassMember, JsGetterObjectMember, JsMethodClassMember,
     JsMethodObjectMember,
 };
-use biome_rowan::{declare_node_union, AstNode, SyntaxNodeOptionExt, TextRange};
+use biome_rowan::{declare_node_union, AstNode, SyntaxNode, SyntaxNodeOptionExt, TextRange};
 
 declare_lint_rule! {
     /// Require explicit return types on functions and class methods.
@@ -168,6 +170,38 @@ declare_lint_rule! {
     /// }
     /// ```
     ///
+    /// The following patterns are considered correct for type annotations on variables in function expressions:
+    ///
+    /// ```ts
+    /// // A function with a type assertion using `as`
+    /// const asTyped = (() => '') as () => string;
+    /// ```
+    ///
+    /// ```ts
+    /// // A function with a type assertion using `<>`
+    /// const castTyped = <() => string>(() => '');
+    /// ```
+    ///
+    /// ```ts
+    /// // A variable declarator with a type annotation.
+    /// type FuncType = () => string;
+    /// const arrowFn: FuncType = () => 'test';
+    /// ```
+    ///
+    /// ```ts
+    /// // A function is a default parameter with a type annotation
+    /// type CallBack = () => void;
+    /// const f = (gotcha: CallBack = () => { }): void => { };
+    /// ```
+    ///
+    /// ```ts
+    /// // A class property with a type annotation
+    /// type MethodType = () => void;
+    /// class App {
+    ///     private method: MethodType = () => { };
+    /// }
+    /// ```
+    ///
     pub UseExplicitFunctionReturnType {
         version: "1.9.3",
         name: "useExplicitFunctionReturnType",
@@ -204,14 +238,22 @@ impl Rule for UseExplicitFunctionReturnType {
                     return None;
                 }
 
-                if is_function_used_in_argument_or_expression_list(func) {
+                if is_iife(func) {
+                    return None;
+                }
+
+                if is_function_used_in_argument_or_array(func) {
                     return None;
                 }
 
                 if is_higher_order_function(func) {
                     return None;
                 }
 
+                if is_typed_function_expressions(func) {
+                    return None;
+                }
+
                 let func_range = func.syntax().text_range();
                 if let Ok(Some(AnyJsBinding::JsIdentifierBinding(id))) = func.id() {
                     return Some(TextRange::new(
@@ -313,22 +355,27 @@ fn is_direct_const_assertion_in_arrow_functions(func: &AnyJsFunction) -> bool {
 /// JS_ARRAY_ELEMENT_LIST:
 /// - `[function () {}, () => {}];`
 ///
-/// JS_PARENTHESIZED_EXPRESSION:
-/// - `(function () {});`
-/// - `(() => {})();`
-fn is_function_used_in_argument_or_expression_list(func: &AnyJsFunction) -> bool {
+fn is_function_used_in_argument_or_array(func: &AnyJsFunction) -> bool {
     matches!(
         func.syntax().parent().kind(),
-        Some(
-            JsSyntaxKind::JS_CALL_ARGUMENT_LIST
-                | JsSyntaxKind::JS_ARRAY_ELEMENT_LIST
-                // We include JS_PARENTHESIZED_EXPRESSION for IIFE (Immediately Invoked Function Expressions).
-                // We also assume that the parent of the parent is a call expression.
-                | JsSyntaxKind::JS_PARENTHESIZED_EXPRESSION
-        )
+        Some(JsSyntaxKind::JS_CALL_ARGUMENT_LIST | JsSyntaxKind::JS_ARRAY_ELEMENT_LIST)
     )
 }
 
+/// Checks if a function is an IIFE (Immediately Invoked Function Expressions)
+///
+/// # Examples
+///
+/// ```typescript
+/// (function () {});
+/// (() => {})();
+/// ```
+fn is_iife(func: &AnyJsFunction) -> bool {
+    func.parent::<JsParenthesizedExpression>()
+        .and_then(|expr| expr.parent::<JsCallExpression>())
+        .is_some()
+}
+
 /// Checks whether the given function is a higher-order function, i.e., a function
 /// that returns another function either directly in its body or as an expression.
 ///
@@ -384,11 +431,122 @@ fn is_first_statement_function_return(statements: JsStatementList) -> bool {
                 None
             }
         })
-        .map_or(false, |args| {
+        .is_some_and(|args| {
             matches!(
                 args,
                 AnyJsExpression::JsFunctionExpression(_)
                     | AnyJsExpression::JsArrowFunctionExpression(_)
             )
         })
 }
+
+/// Checks if a given function expression has a type annotation.
+fn is_typed_function_expressions(func: &AnyJsFunction) -> bool {
+    let syntax = func.syntax();
+    is_type_assertion(syntax)
+        || is_variable_declarator_with_type_annotation(syntax)
+        || is_default_function_parameter_with_type_annotation(syntax)
+        || is_class_property_with_type_annotation(syntax)
+        || is_property_of_object_with_type(syntax)
+}
+
+/// Checks if a function is a variable declarator with a type annotation.
+///
+/// # Examples
+///
+/// ```typescript
+/// type FuncType = () => string;
+/// const arrowFn: FuncType = () => 'test';
+/// ```
+fn is_variable_declarator_with_type_annotation(syntax: &SyntaxNode<JsLanguage>) -> bool {
+    syntax
+        .parent()
+        .and_then(JsInitializerClause::cast)
+        .and_then(|init| init.parent::<JsVariableDeclarator>())
+        .is_some_and(|decl| decl.variable_annotation().is_some())
+}
+
+/// Checks if a function is a default parameter with a type annotation.
+///
+/// # Examples
+///
+/// ```typescript
+/// type CallBack = () => void;
+/// const f = (gotcha: CallBack = () => { }): void => { };
+/// ```
+fn is_default_function_parameter_with_type_annotation(syntax: &SyntaxNode<JsLanguage>) -> bool {
+    syntax
+        .parent()
+        .and_then(JsInitializerClause::cast)
+        .and_then(|init| init.parent::<JsFormalParameter>())
+        .is_some_and(|param| param.type_annotation().is_some())
+}
+
+/// Checks if a function is a class property with a type annotation.
+///
+/// # Examples
+///
+/// ```typescript
+/// type MethodType = () => void;
+/// class App {
+///     private method: MethodType = () => { };
+/// }
+/// ```
+fn is_class_property_with_type_annotation(syntax: &SyntaxNode<JsLanguage>) -> bool {
+    syntax
+        .parent()
+        .and_then(JsInitializerClause::cast)
+        .and_then(|init| init.parent::<JsPropertyClassMember>())
+        .is_some_and(|prop| prop.property_annotation().is_some())
+}
+
+/// Checks if a function is a property or a nested property of a typed object.
+///
+/// # Examples
+///
+/// ```typescript
+/// const x: Foo = { prop: () => {} }
+/// const x = { prop: () => {} } as Foo
+/// const x = <Foo>{ prop: () => {} }
+/// const x: Foo = { bar: { prop: () => {} } }
+/// ```
+fn is_property_of_object_with_type(syntax: &SyntaxNode<JsLanguage>) -> bool {
+    syntax
+        .parent()
+        .and_then(JsPropertyObjectMember::cast)
+        .and_then(|prop| prop.syntax().grand_parent())
+        .and_then(JsObjectExpression::cast)
+        .is_some_and(|obj_expression| {
+            let obj_syntax = obj_expression.syntax();
+            is_type_assertion(obj_syntax)
+                || is_variable_declarator_with_type_annotation(obj_syntax)
+                || is_property_of_object_with_type(obj_syntax)
+        })
+}
+
+/// Checks if a function has a type assertion.
+///
+/// # Examples
+///
+/// ```typescript
+/// const asTyped = (() => '') as () => string;
+/// const castTyped = <() => string>(() => '');
+/// ```
+fn is_type_assertion(syntax: &SyntaxNode<JsLanguage>) -> bool {
+    fn is_assertion_kind(kind: JsSyntaxKind) -> bool {
+        matches!(
+            kind,
+            JsSyntaxKind::TS_AS_EXPRESSION | JsSyntaxKind::TS_TYPE_ASSERTION_EXPRESSION
+        )
+    }
+
+    syntax.parent().map_or(false, |parent| {
+        if parent.kind() == JsSyntaxKind::JS_PARENTHESIZED_EXPRESSION {
+            parent
+                .parent()
+                .is_some_and(|grandparent| is_assertion_kind(grandparent.kind()))
+        } else {
+            is_assertion_kind(parent.kind())
+        }
+    })
+}

crates/biome_js_analyze/tests/specs/nursery/useExplicitFunctionReturnType/invalid.ts

@@ -89,4 +89,7 @@ function fn() {
   return function (): string {
       return str;
   };
-}
+}
+
+const x = { prop: () => {} }
+const x = { bar: { prop: () => {} } }

crates/biome_js_analyze/tests/specs/nursery/useExplicitFunctionReturnType/invalid.ts.snap

@@ -96,6 +96,9 @@ function fn() {
       return str;
   };
 }
+
+const x = { prop: () => {} }
+const x = { bar: { prop: () => {} } }
 ```
 
 # Diagnostics
@@ -527,3 +530,37 @@ invalid.ts:85:2 lint/nursery/useExplicitFunctionReturnType ━━━━━━━
 
 
 ```
+
+```
+invalid.ts:94:19 lint/nursery/useExplicitFunctionReturnType ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━
+
+  ! Missing return type on function.
+
+    92 │ }
+    93 │ 
+  > 94 │ const x = { prop: () => {} }
+       │                   ^^^^^^^^^
+    95 │ const x = { bar: { prop: () => {} } }
+
+  i Declaring the return type makes the code self-documenting and can speed up TypeScript type checking.
+
+  i Add a return type annotation.
+
+
+```
+
+```
+invalid.ts:95:26 lint/nursery/useExplicitFunctionReturnType ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━
+
+  ! Missing return type on function.
+
+    94 │ const x = { prop: () => {} }
+  > 95 │ const x = { bar: { prop: () => {} } }
+       │                          ^^^^^^^^^
+
+  i Declaring the return type makes the code self-documenting and can speed up TypeScript type checking.
+
+  i Add a return type annotation.
+
+
+```

crates/biome_js_analyze/tests/specs/nursery/useExplicitFunctionReturnType/valid.ts

@@ -47,6 +47,8 @@ node.addEventListener('click', function () {});
 const foo = arr.map(i => i * i);
 fn(() => {});
 fn(function () {});
+new Promise(resolve => {});
+new Foo(1, () => {});
 [function () {}, () => {}];
 (function () {
   console.log("This is an IIFE");
@@ -62,4 +64,42 @@ const arrowFn = () => (): void => {};
 const arrowFn = () => function(): void {}
 const arrowFn = () => {
   return (): void => { };
-}
+}
+
+
+// type assertion
+const asTyped = (() => '') as () => string;
+const castTyped = <() => string>(() => '');
+
+// variable declarator with a type annotation 
+type FuncType = () => string;
+const arrowFn: FuncType = () => 'test';
+const funcExpr: FuncType = function () {
+  return 'test';
+};
+
+// default parameter with a type annotation
+type CallBack = () => void;
+const f = (gotcha: CallBack = () => { }): void => { };
+function f(gotcha: CallBack = () => {}): void {}
+
+// class property with a type annotation
+type MethodType = () => void;
+class App {
+    private method: MethodType = () => { };
+}
+
+// function as a property or a nested property of a typed object
+const x: Foo = { prop: () => {} }
+const x = { prop: () => {} } as Foo
+const x = <Foo>{ prop: () => {} }
+
+const x: Foo = { bar: { prop: () => {} } }
+
+class Accumulator {
+  private count: number = 0;
+  public accumulate(fn: () => number): void {
+      this.count += fn();
+  }
+}
+new Accumulator().accumulate(() => 1);