Fix regression: preserve previous inJsxChild state in nested transforms

The previous fix incorrectly cleared the inJsxChild flag when transforming
nested JSX children. When transforming a nested child element, the defer
statement would set inJsxChild back to false, causing the parent element
to not receive the EFStartOnNewLine flag.

Fixed by saving and restoring the previous value of inJsxChild instead of
unconditionally setting it to false. This ensures that the flag state is
properly maintained across nested transformations.

Also fixed the logic for adding EFStartOnNewLine to children: the flag
should only be added when there are multiple children AFTER filtering out
null/whitespace children, not before. This matches TypeScript's behavior
where a single child expression does not get a newline, but multiple
children do.

These fixes resolve the regression in commentsOnJSXExpressionsArePreserved
tests, where a single JSX expression child was incorrectly getting a newline.

Co-authored-by: jakebailey <[email protected]>

Author: Copilot

internal/transformers/jsxtransforms/jsx.go

@@ -319,8 +319,9 @@ func (tx *JSXTransformer) convertJsxChildrenToChildrenPropObject(children []*ast
 }
 
 func (tx *JSXTransformer) transformJsxChildToExpression(node *ast.Node) *ast.Node {
+	prev := tx.inJsxChild
 	tx.setInChild(true)
-	defer tx.setInChild(false)
+	defer tx.setInChild(prev)
 	return tx.Visitor().Visit(node)
 }
 
@@ -688,14 +689,18 @@ func (tx *JSXTransformer) visitJsxOpeningLikeElementCreateElement(element *ast.N
 		for _, c := range children.Nodes {
 			res := tx.transformJsxChildToExpression(c)
 			if res != nil {
-				if len(children.Nodes) > 1 {
-					tx.EmitContext().AddEmitFlags(res, printer.EFStartOnNewLine)
-				}
 				newChildren = append(newChildren, res)
 			}
 		}
 	}
 
+	// Add StartOnNewLine flag only if there are multiple actual children (after filtering)
+	if len(newChildren) > 1 {
+		for _, child := range newChildren {
+			tx.EmitContext().AddEmitFlags(child, printer.EFStartOnNewLine)
+		}
+	}
+
 	args := make([]*ast.Expression, 0, len(newChildren)+2)
 	args = append(args, tagName)
 	args = append(args, objectProperties)
@@ -725,14 +730,18 @@ func (tx *JSXTransformer) visitJsxOpeningFragmentCreateElement(fragment *ast.Jsx
 		for _, c := range children.Nodes {
 			res := tx.transformJsxChildToExpression(c)
 			if res != nil {
-				if len(children.Nodes) > 1 {
-					tx.EmitContext().AddEmitFlags(res, printer.EFStartOnNewLine)
-				}
 				newChildren = append(newChildren, res)
 			}
 		}
 	}
 
+	// Add StartOnNewLine flag only if there are multiple actual children (after filtering)
+	if len(newChildren) > 1 {
+		for _, child := range newChildren {
+			tx.EmitContext().AddEmitFlags(child, printer.EFStartOnNewLine)
+		}
+	}
+
 	args := make([]*ast.Expression, 0, len(newChildren)+2)
 	args = append(args, tagName)
 	args = append(args, tx.Factory().NewKeywordExpression(ast.KindNullKeyword))

testdata/baselines/reference/compiler/jsxUnicodeEscapeSequence.js

@@ -34,7 +34,9 @@ export const InlineUnicodeChar = () => {
 };
 export const StandaloneUnicodeChar = () => {
     // This should reproduce the issue - unicode character on its own line
-    return (_jsxs("div", { children: [_jsx("span", { children: "\u26A0" }), "\u26A0"] }));
+    return (_jsxs("div", { children: [
+            _jsx("span", { children: "\u26A0" }),
+            "\u26A0"] }));
 };
 export const MultipleUnicodeChars = () => {
     // Test multiple unicode characters

testdata/baselines/reference/submodule/compiler/commentsOnJSXExpressionsArePreserved(jsx=react,module=commonjs,moduledetection=auto).js

@@ -25,7 +25,6 @@ class Component {
 //// [commentsOnJSXExpressionsArePreserved.js]
 class Component {
     render() {
-        return React.createElement("div", null,
-            null /* preserved */);
+        return React.createElement("div", null, null /* preserved */);
     }
 }

testdata/baselines/reference/submodule/compiler/commentsOnJSXExpressionsArePreserved(jsx=react,module=commonjs,moduledetection=auto).js.diff

@@ -27,7 +27,6 @@ class Component {
 Object.defineProperty(exports, "__esModule", { value: true });
 class Component {
     render() {
-        return React.createElement("div", null,
-            null /* preserved */);
+        return React.createElement("div", null, null /* preserved */);
     }
 }

testdata/baselines/reference/submodule/compiler/commentsOnJSXExpressionsArePreserved(jsx=react,module=commonjs,moduledetection=force).js

@@ -25,7 +25,6 @@ class Component {
 //// [commentsOnJSXExpressionsArePreserved.js]
 class Component {
     render() {
-        return React.createElement("div", null,
-            null /* preserved */);
+        return React.createElement("div", null, null /* preserved */);
     }
 }

testdata/baselines/reference/submodule/compiler/commentsOnJSXExpressionsArePreserved(jsx=react,module=commonjs,moduledetection=force).js.diff

@@ -90,8 +90,7 @@ function Blah(props) {
     return React.createElement(React.Fragment, null);
 }
 // Incompatible child.
-var a = React.createElement(Blah, null,
-    x => x);
+var a = React.createElement(Blah, null, x => x);
 // Blah components don't accept text as child elements
 var a = React.createElement(Blah, null, "Hello unexpected text!");
 // Blah components don't accept multiple children.
@@ -102,8 +101,7 @@ function Blah2(props) {
     return React.createElement(React.Fragment, null);
 }
 // Incompatible child.
-var a = React.createElement(Blah2, null,
-    x => x);
+var a = React.createElement(Blah2, null, x => x);
 // Blah2 components don't accept text as child elements
 var a = React.createElement(Blah2, null, "Hello unexpected text!");
 // Blah2 components don't accept multiple children of the wrong type.
@@ -114,8 +112,7 @@ function Blah3(props) {
     return React.createElement(React.Fragment, null);
 }
 // Incompatible child.
-var a = React.createElement(Blah3, null,
-    x => x);
+var a = React.createElement(Blah3, null, x => x);
 // Blah3 components don't accept text as child elements
 var a = React.createElement(Blah3, null, "Hello unexpected text!");
 // Blah3 components don't accept multiple children of the wrong type.

testdata/baselines/reference/submodule/compiler/commentsOnJSXExpressionsArePreserved(jsx=react,module=commonjs,moduledetection=legacy).js

@@ -8,31 +8,4 @@
 +const React = require("react");
  function Blah(props) {
      return React.createElement(React.Fragment, null);
- }
- // Incompatible child.
--var a = React.createElement(Blah, null, x => x);
-+var a = React.createElement(Blah, null,
-+    x => x);
- // Blah components don't accept text as child elements
- var a = React.createElement(Blah, null, "Hello unexpected text!");
- // Blah components don't accept multiple children.
-@@= skipped -16, +17 lines =@@
-     return React.createElement(React.Fragment, null);
- }
- // Incompatible child.
--var a = React.createElement(Blah2, null, x => x);
-+var a = React.createElement(Blah2, null,
-+    x => x);
- // Blah2 components don't accept text as child elements
- var a = React.createElement(Blah2, null, "Hello unexpected text!");
- // Blah2 components don't accept multiple children of the wrong type.
-@@= skipped -11, +12 lines =@@
-     return React.createElement(React.Fragment, null);
- }
- // Incompatible child.
--var a = React.createElement(Blah3, null, x => x);
-+var a = React.createElement(Blah3, null,
-+    x => x);
- // Blah3 components don't accept text as child elements
- var a = React.createElement(Blah3, null, "Hello unexpected text!");
- // Blah3 components don't accept multiple children of the wrong type.
+ }