Repository: microsoft/typescript-go (go)

Issue:
[bug] support `null`ing out extended tsconfig.json properties
simple repro:

```jsonc
// tsconfig.base.json
{
  "compilerOptions": {
    "types": []
  },
}

// tsconfig.json
{
  "compilerOptions": {
    "types": null
  },
  "extends": "./tsconfig.base.json"
}
```

then run the below and note that `compilerOptions.types` is still set to the empty array, `[]`.

```
$ built/local/tsgo -p path/to/tsconfig.json -showConfig
{
  ...
  "types": [],
  ...
}
```

Pull request description:
Fix tsconfig null override for extended configurations
This PR fixes support for "nulling out" extended tsconfig.json properties by setting them to `null`.

## Problem

When a tsconfig.json extends another tsconfig.json and tries to set a property to `null` to clear it, the current implementation doesn't handle this correctly. Consider:

```jsonc
// tsconfig-base.json
{
  "compilerOptions": {
    "types": ["node"]
  }
}

// tsconfig.json
{
  "compilerOptions": {
    "types": null
  },
  "extends": "./tsconfig-base.json"
}
```

**Before this fix:**
```bash
$ tsgo -p . --showConfig
{
  "types": ["node"],  // ❌ Still shows inherited value
  "configFilePath": "./tsconfig.json"
}
```

**After this fix:**
```bash
$ tsgo -p . --showConfig
{
  "configFilePath": "./tsconfig.json"
  // ✅ types field is omitted (successfully nulled out)
}
```

## Root Cause

The issue was in the `mergeCompilerOptions` function which uses `IsZero()` to check if a field should be merged. In Go:

- `null` in JSON → Go `nil` slice → `IsZero()` returns `true` → merge skips it
- `[]` in JSON → Go empty slice → `IsZero()` returns `false` → merge applies it
- Not set → Go `nil` slice → `IsZero()` returns `true` → merge skips it

This meant the merge function couldn't distinguish between "not set" and "explicitly set to null".

## Solution

- **Added `mergeCompilerOptionsWithRaw`**: A new merge function that's aware of explicitly set null values in the raw JSON
- **Added `handleExplicitNullValues`**: Processes raw JSON to detect fields explicitly set to `null` and applies them as overrides
- **Updated tsconfig extending logic**: Uses the new merge function during configuration extension

## Test Coverage

Added comprehensive test cases:
- Single field null override (`tsconfigNullOverride.ts`)
- Multiple field null override (`tsconfigNullOverrideMultipleFields.ts`)
- Verified existing behavior still works (empty arrays, inheritance, etc.)

All existing tests pass, confirming backward compatibility.

Fixes #768.

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Patch:
diff --git a/internal/checker/checker.go b/internal/checker/checker.go
index 51f002749..887bd0ed2 100644
--- a/internal/checker/checker.go
+++ b/internal/checker/checker.go
@@ -30572,9 +30572,9 @@ func (c *Checker) GetTypeAtLocation(node *ast.Node) *Type {
 	return c.getTypeOfNode(node)
 }
 
-func (c *Checker) GetEmitResolver(file *ast.SourceFile) *emitResolver {
+func (c *Checker) GetEmitResolver() *emitResolver {
 	c.emitResolverOnce.Do(func() {
-		c.emitResolver = &emitResolver{checker: c}
+		c.emitResolver = newEmitResolver(c)
 	})
 
 	return c.emitResolver
diff --git a/internal/checker/emitresolver.go b/internal/checker/emitresolver.go
index 4be45bc03..deca1d3a4 100644
--- a/internal/checker/emitresolver.go
+++ b/internal/checker/emitresolver.go
@@ -14,7 +14,7 @@ import (
 	"github.com/microsoft/typescript-go/internal/printer"
 )
 
-var _ printer.EmitResolver = &emitResolver{}
+var _ printer.EmitResolver = (*emitResolver)(nil)
 
 // Links for jsx
 type JSXLinks struct {
@@ -35,12 +35,20 @@ type emitResolver struct {
 	checker                 *Checker
 	checkerMu               sync.Mutex
 	isValueAliasDeclaration func(node *ast.Node) bool
+	aliasMarkingVisitor     func(node *ast.Node) bool
 	referenceResolver       binder.ReferenceResolver
 	jsxLinks                core.LinkStore[*ast.Node, JSXLinks]
 	declarationLinks        core.LinkStore[*ast.Node, DeclarationLinks]
 	declarationFileLinks    core.LinkStore[*ast.Node, DeclarationFileLinks]
 }
 
+func newEmitResolver(checker *Checker) *emitResolver {
+	e := &emitResolver{checker: checker}
+	e.isValueAliasDeclaration = e.isValueAliasDeclarationWorker
+	e.aliasMarkingVisitor = e.aliasMarkingVisitorWorker
+	return e
+}
+
 func (r *emitResolver) GetJsxFactoryEntity(location *ast.Node) *ast.Node {
 	r.checkerMu.Lock()
 	defer r.checkerMu.Unlock()
@@ -227,7 +235,7 @@ func (r *emitResolver) PrecalculateDeclarationEmitVisibility(file *ast.SourceFil
 	file.AsNode().ForEachChild(r.aliasMarkingVisitor)
 }
 
-func (r *emitResolver) aliasMarkingVisitor(node *ast.Node) bool {
+func (r *emitResolver) aliasMarkingVisitorWorker(node *ast.Node) bool {
 	switch node.Kind {
 	case ast.KindExportAssignment, ast.KindJSExportAssignment:
 		if node.AsExportAssignment().Expression.Kind == ast.KindIdentifier {
@@ -670,9 +678,6 @@ func (r *emitResolver) isValueAliasDeclarationWorker(node *ast.Node) bool {
 		return symbol != nil && r.isAliasResolvedToValue(symbol, true /*excludeTypeOnlyValues*/)
 	case ast.KindExportDeclaration:
 		exportClause := node.AsExportDeclaration().ExportClause
-		if r.isValueAliasDeclaration == nil {
-			r.isValueAliasDeclaration = r.isValueAliasDeclarationWorker
-		}
 		return exportClause != nil && (ast.IsNamespaceExport(exportClause) ||
 			core.Some(exportClause.AsNamedExports().Elements.Nodes, r.isValueAliasDeclaration))
 	case ast.KindExportAssignment, ast.KindJSExportAssignment:
diff --git a/internal/checker/nodebuilderimpl.go b/internal/checker/nodebuilderimpl.go
index 72a4cd1f9..015d42f55 100644
--- a/internal/checker/nodebuilderimpl.go
+++ b/internal/checker/nodebuilderimpl.go
@@ -1450,7 +1450,7 @@ func (b *nodeBuilderImpl) symbolToParameterDeclaration(parameterSymbol *ast.Symb
 	}
 	name := b.parameterToParameterDeclarationName(parameterSymbol, parameterDeclaration)
 	// TODO: isOptionalParameter on emit resolver here is silly - hoist to checker and reexpose on emit resolver?
-	isOptional := parameterDeclaration != nil && b.ch.GetEmitResolver(nil).isOptionalParameter(parameterDeclaration) || parameterSymbol.CheckFlags&ast.CheckFlagsOptionalParameter != 0
+	isOptional := parameterDeclaration != nil && b.ch.GetEmitResolver().isOptionalParameter(parameterDeclaration) || parameterSymbol.CheckFlags&ast.CheckFlagsOptionalParameter != 0
 	var questionToken *ast.Node
 	if isOptional {
 		questionToken = b.f.NewToken(ast.KindQuestionToken)
@@ -1887,7 +1887,7 @@ func (b *nodeBuilderImpl) serializeTypeForDeclaration(declaration *ast.Declarati
 			}
 		}
 		// !!! TODO: JSDoc, getEmitResolver call is unfortunate layering for the helper - hoist it into checker
-		addUndefinedForParameter := declaration != nil && (ast.IsParameter(declaration) /*|| ast.IsJSDocParameterTag(declaration)*/) && b.ch.GetEmitResolver(nil).requiresAddingImplicitUndefined(declaration, symbol, b.ctx.enclosingDeclaration)
+		addUndefinedForParameter := declaration != nil && (ast.IsParameter(declaration) /*|| ast.IsJSDocParameterTag(declaration)*/) && b.ch.GetEmitResolver().requiresAddingImplicitUndefined(declaration, symbol, b.ctx.enclosingDeclaration)
 		if addUndefinedForParameter {
 			t = b.ch.getOptionalType(t, false)
 		}
diff --git a/internal/checker/symbolaccessibility.go b/internal/checker/symbolaccessibility.go
index c34d1d3bd..cd47e3c33 100644
--- a/internal/checker/symbolaccessibility.go
+++ b/internal/checker/symbolaccessibility.go
@@ -38,7 +38,7 @@ func (ch *Checker) IsAnySymbolAccessible(symbols []*ast.Symbol, enclosingDeclara
 		if len(accessibleSymbolChain) > 0 {
 			hadAccessibleChain = symbol
 			// TODO: going through emit resolver here is weird. Relayer these APIs.
-			hasAccessibleDeclarations := ch.GetEmitResolver(nil).hasVisibleDeclarations(accessibleSymbolChain[0], shouldComputeAliasesToMakeVisible)
+			hasAccessibleDeclarations := ch.GetEmitResolver().hasVisibleDeclarations(accessibleSymbolChain[0], shouldComputeAliasesToMakeVisible)
 			if hasAccessibleDeclarations != nil {
 				return hasAccessibleDeclarations
 			}
diff --git a/internal/compiler/emitHost.go b/internal/compiler/emitHost.go
index 9161e1f35..d1db60a4d 100644
--- a/internal/compiler/emitHost.go
+++ b/internal/compiler/emitHost.go
@@ -32,14 +32,22 @@ type EmitHost interface {
 	GetCurrentDirectory() string
 	CommonSourceDirectory() string
 	IsEmitBlocked(file string) bool
-	GetEmitResolver(file *ast.SourceFile) printer.EmitResolver
 }
 
 var _ EmitHost = (*emitHost)(nil)
 
 // NOTE: emitHost operations must be thread-safe
 type emitHost struct {
-	program *Program
+	program      *Program
+	emitResolver printer.EmitResolver
+}
+
+func newEmitHost(ctx context.Context, program *Program, file *ast.SourceFile) (*emitHost, func()) {
+	checker, done := program.GetTypeCheckerForFile(ctx, file)
+	return &emitHost{
+		program:      program,
+		emitResolver: checker.GetEmitResolver(),
+	}, done
 }
 
 func (host *emitHost) GetModeForUsageLocation(file ast.HasFileName, moduleSpecifier *ast.StringLiteralLike) core.ResolutionMode {
@@ -83,7 +91,7 @@ func (host *emitHost) GetRedirectTargets(path tspath.Path) []string {
 }
 
 func (host *emitHost) GetEffectiveDeclarationFlags(node *ast.Node, flags ast.ModifierFlags) ast.ModifierFlags {
-	return host.GetEmitResolver(ast.GetSourceFileOfNode(node)).GetEffectiveDeclarationFlags(node, flags)
+	return host.GetEmitResolver().GetEffectiveDeclarationFlags(node, flags)
 }
 
 func (host *emitHost) GetOutputPathsFor(file *ast.SourceFile, forceDtsPaths bool) declarations.OutputPaths {
@@ -92,7 +100,7 @@ func (host *emitHost) GetOutputPathsFor(file *ast.SourceFile, forceDtsPaths bool
 }
 
 func (host *emitHost) GetResolutionModeOverride(node *ast.Node) core.ResolutionMode {
-	return host.GetEmitResolver(ast.GetSourceFileOfNode(node)).GetResolutionModeOverride(node)
+	return host.GetEmitResolver().GetResolutionModeOverride(node)
 }
 
 func (host *emitHost) GetSourceFileFromReference(origin *ast.SourceFile, ref *ast.FileReference) *ast.SourceFile {
@@ -116,13 +124,8 @@ func (host *emitHost) WriteFile(fileName string, text string, writeByteOrderMark
 	return host.program.Host().FS().WriteFile(fileName, text, writeByteOrderMark)
 }
 
-func (host *emitHost) GetEmitResolver(file *ast.SourceFile) printer.EmitResolver {
-	// The context and done function don't matter in tsc, currently the only caller of this function.
-	// But if this ever gets used by LSP code, we'll need to thread the context properly and pass the
-	// done function to the caller to ensure resources are cleaned up at the end of the request.
-	checker, done := host.program.GetTypeCheckerForFile(context.TODO(), file)
-	defer done()
-	return checker.GetEmitResolver(file)
+func (host *emitHost) GetEmitResolver() printer.EmitResolver {
+	return host.emitResolver
 }
 
 func (host *emitHost) IsSourceFileFromExternalLibrary(file *ast.SourceFile) bool {
diff --git a/internal/compiler/emitter.go b/internal/compiler/emitter.go
index 9cd30237b..b031647c2 100644
--- a/internal/compiler/emitter.go
+++ b/internal/compiler/emitter.go
@@ -49,9 +49,8 @@ func (e *emitter) emit() {
 	e.emitBuildInfo(e.paths.BuildInfoPath())
 }
 
-func (e *emitter) getDeclarationTransformers(emitContext *printer.EmitContext, sourceFile *ast.SourceFile, declarationFilePath string, declarationMapPath string) []*declarations.DeclarationTransformer {
-	emitResolver := e.host.GetEmitResolver(sourceFile)
-	transform := declarations.NewDeclarationTransformer(e.host, emitResolver, emitContext, e.host.Options(), declarationFilePath, declarationMapPath)
+func (e *emitter) getDeclarationTransformers(emitContext *printer.EmitContext, declarationFilePath string, declarationMapPath string) []*declarations.DeclarationTransformer {
+	transform := declarations.NewDeclarationTransformer(e.host, emitContext, e.host.Options(), declarationFilePath, declarationMapPath)
 	return []*declarations.DeclarationTransformer{transform}
 }
 
@@ -86,7 +85,7 @@ func getScriptTransformers(emitContext *printer.EmitContext, host printer.EmitHo
 	var emitResolver printer.EmitResolver
 	var referenceResolver binder.ReferenceResolver
 	if importElisionEnabled || options.GetJSXTransformEnabled() {
-		emitResolver = host.GetEmitResolver(sourceFile)
+		emitResolver = host.GetEmitResolver()
 		emitResolver.MarkLinkedReferencesRecursively(sourceFile)
 		referenceResolver = emitResolver
 	} else {
@@ -179,7 +178,7 @@ func (e *emitter) emitDeclarationFile(sourceFile *ast.SourceFile, declarationFil
 	var diags []*ast.Diagnostic
 	emitContext, putEmitContext := printer.GetEmitContext()
 	defer putEmitContext()
-	for _, transformer := range e.getDeclarationTransformers(emitContext, sourceFile, declarationFilePath, declarationMapPath) {
+	for _, transformer := range e.getDeclarationTransformers(emitContext, declarationFilePath, declarationMapPath) {
 		sourceFile = transformer.TransformSourceFile(sourceFile)
 		diags = append(diags, transformer.GetDiagnostics()...)
 	}
@@ -378,6 +377,7 @@ type SourceFileMayBeEmittedHost interface {
 	IsSourceFileFromExternalLibrary(file *ast.SourceFile) bool
 	GetCurrentDirectory() string
 	UseCaseSensitiveFileNames() bool
+	SourceFiles() []*ast.SourceFile
 }
 
 func sourceFileMayBeEmitted(sourceFile *ast.SourceFile, host SourceFileMayBeEmittedHost, forceDtsEmit bool) bool {
@@ -435,7 +435,7 @@ func sourceFileMayBeEmitted(sourceFile *ast.SourceFile, host SourceFileMayBeEmit
 	return true
 }
 
-func getSourceFilesToEmit(host printer.EmitHost, targetSourceFile *ast.SourceFile, forceDtsEmit bool) []*ast.SourceFile {
+func getSourceFilesToEmit(host SourceFileMayBeEmittedHost, targetSourceFile *ast.SourceFile, forceDtsEmit bool) []*ast.SourceFile {
 	// !!! outFile not yet implemented, may be deprecated
 	var sourceFiles []*ast.SourceFile
 	if targetSourceFile != nil {
@@ -452,13 +452,13 @@ func isSourceFileNotJson(file *ast.SourceFile) bool {
 	return !ast.IsJsonSourceFile(file)
 }
 
-func getDeclarationDiagnostics(host EmitHost, resolver printer.EmitResolver, file *ast.SourceFile) []*ast.Diagnostic {
+func getDeclarationDiagnostics(host EmitHost, file *ast.SourceFile) []*ast.Diagnostic {
 	fullFiles := core.Filter(getSourceFilesToEmit(host, file, false), isSourceFileNotJson)
 	if !core.Some(fullFiles, func(f *ast.SourceFile) bool { return f == file }) {
 		return []*ast.Diagnostic{}
 	}
 	options := host.Options()
-	transform := declarations.NewDeclarationTransformer(host, resolver, nil, options, "", "")
+	transform := declarations.NewDeclarationTransformer(host, nil, options, "", "")
 	transform.TransformSourceFile(file)
 	return transform.GetDiagnostics()
 }
diff --git a/internal/compiler/program.go b/internal/compiler/program.go
index 7dbd620a6..fa2ac6e90 100644
--- a/internal/compiler/program.go
+++ b/internal/compiler/program.go
@@ -449,7 +449,7 @@ func (p *Program) getSemanticDiagnosticsForFile(ctx context.Context, sourceFile
 	return filtered
 }
 
-func (p *Program) getDeclarationDiagnosticsForFile(_ctx context.Context, sourceFile *ast.SourceFile) []*ast.Diagnostic {
+func (p *Program) getDeclarationDiagnosticsForFile(ctx context.Context, sourceFile *ast.SourceFile) []*ast.Diagnostic {
 	if sourceFile.IsDeclarationFile {
 		return []*ast.Diagnostic{}
 	}
@@ -458,8 +458,9 @@ func (p *Program) getDeclarationDiagnosticsForFile(_ctx context.Context, sourceF
 		return cached
 	}
 
-	host := &emitHost{program: p}
-	diagnostics := getDeclarationDiagnostics(host, host.GetEmitResolver(sourceFile), sourceFile)
+	host, done := newEmitHost(ctx, p, sourceFile)
+	defer done()
+	diagnostics := getDeclarationDiagnostics(host, sourceFile)
 	diagnostics, _ = p.declarationDiagnosticCache.LoadOrStore(sourceFile, diagnostics)
 	return diagnostics
 }
@@ -651,9 +652,8 @@ func (p *Program) CommonSourceDirectory() string {
 			p.Options(),
 			func() []string {
 				var files []string
-				host := &emitHost{program: p}
 				for _, file := range p.files {
-					if sourceFileMayBeEmitted(file, host, false /*forceDtsEmit*/) {
+					if sourceFileMayBeEmitted(file, p, false /*forceDtsEmit*/) {
 						files = append(files, file.FileName())
 					}
 				}
@@ -688,20 +688,17 @@ func (p *Program) Emit(options EmitOptions) *EmitResult {
 	// !!! performance measurement
 	p.BindSourceFiles()
 
-	host := &emitHost{program: p}
-
 	writerPool := &sync.Pool{
 		New: func() any {
-			return printer.NewTextWriter(host.Options().NewLine.GetNewLineCharacter())
+			return printer.NewTextWriter(p.Options().NewLine.GetNewLineCharacter())
 		},
 	}
 	wg := core.NewWorkGroup(p.singleThreaded())
 	var emitters []*emitter
-	sourceFiles := getSourceFilesToEmit(host, options.TargetSourceFile, options.forceDtsEmit)
+	sourceFiles := getSourceFilesToEmit(p, options.TargetSourceFile, options.forceDtsEmit)
 
 	for _, sourceFile := range sourceFiles {
 		emitter := &emitter{
-			host:              host,
 			emittedFilesList:  nil,
 			sourceMapDataList: nil,
 			writer:            nil,
@@ -709,6 +706,10 @@ func (p *Program) Emit(options EmitOptions) *EmitResult {
 		}
 		emitters = append(emitters, emitter)
 		wg.Queue(func() {
+			host, done := newEmitHost(context.TODO(), p, sourceFile)
+			defer done()
+			emitter.host = host
+
 			// take an unused writer
 			writer := writerPool.Get().(printer.EmitTextWriter)
 			writer.Clear()
diff --git a/internal/ls/definition.go b/internal/ls/definition.go
index 6ccc155c2..d90c8c0b6 100644
--- a/internal/ls/definition.go
+++ b/internal/ls/definition.go
@@ -5,6 +5,7 @@ import (
 
 	"github.com/microsoft/typescript-go/internal/ast"
 	"github.com/microsoft/typescript-go/internal/astnav"
+	"github.com/microsoft/typescript-go/internal/checker"
 	"github.com/microsoft/typescript-go/internal/core"
 	"github.com/microsoft/typescript-go/internal/lsp/lsproto"
 	"github.com/microsoft/typescript-go/internal/scanner"
@@ -20,6 +21,14 @@ func (l *LanguageService) ProvideDefinition(ctx context.Context, documentURI lsp
 	checker, done := program.GetTypeCheckerForFile(ctx, file)
 	defer done()
 
+	calledDeclaration := tryGetSignatureDeclaration(checker, node)
+	if calledDeclaration != nil {
+		name := ast.GetNameOfDeclaration(calledDeclaration)
+		if name != nil {
+			return l.createLocationsFromDeclarations([]*ast.Node{name})
+		}
+	}
+
 	if symbol := checker.GetSymbolAtLocation(node); symbol != nil {
 		if symbol.Flags&ast.SymbolFlagsAlias != 0 {
 			if resolved, ok := checker.ResolveAlias(symbol); ok {
@@ -27,17 +36,54 @@ func (l *LanguageService) ProvideDefinition(ctx context.Context, documentURI lsp
 			}
 		}
 
-		locations := make([]lsproto.Location, 0, len(symbol.Declarations))
-		for _, decl := range symbol.Declarations {
-			file := ast.GetSourceFileOfNode(decl)
-			loc := decl.Loc
-			pos := scanner.GetTokenPosOfNode(decl, file, false /*includeJSDoc*/)
-			locations = append(locations, lsproto.Location{
-				Uri:   FileNameToDocumentURI(file.FileName()),
-				Range: l.converters.ToLSPRange(file, core.NewTextRange(pos, loc.End())),
-			})
-		}
-		return &lsproto.Definition{Locations: &locations}, nil
+		return l.createLocationsFromDeclarations(symbol.Declarations)
 	}
 	return nil, nil
 }
+
+func (l *LanguageService) createLocationsFromDeclarations(declarations []*ast.Node) (*lsproto.Definition, error) {
+	locations := make([]lsproto.Location, 0, len(declarations))
+	for _, decl := range declarations {
+		file := ast.GetSourceFileOfNode(decl)
+		loc := decl.Loc
+		pos := scanner.GetTokenPosOfNode(decl, file, false /*includeJSDoc*/)
+		locations = append(locations, lsproto.Location{
+			Uri:   FileNameToDocumentURI(file.FileName()),
+			Range: l.converters.ToLSPRange(file, core.NewTextRange(pos, loc.End())),
+		})
+	}
+	return &lsproto.Definition{Locations: &locations}, nil
+}
+
+/** Returns a CallLikeExpression where `node` is the target being invoked. */
+func getAncestorCallLikeExpression(node *ast.Node) *ast.Node {
+	target := ast.FindAncestor(node, func(n *ast.Node) bool {
+		return !isRightSideOfPropertyAccess(n)
+	})
+
+	callLike := target.Parent
+	if callLike != nil && ast.IsCallLikeExpression(callLike) && ast.GetInvokedExpression(callLike) == target {
+		return callLike
+	}
+
+	return nil
+}
+
+func tryGetSignatureDeclaration(typeChecker *checker.Checker, node *ast.Node) *ast.Node {
+	var signature *checker.Signature
+	callLike := getAncestorCallLikeExpression(node)
+	if callLike != nil {
+		signature = typeChecker.GetResolvedSignature(callLike)
+	}
+
+	// Don't go to a function type, go to the value having that type.
+	var declaration *ast.Node
+	if signature != nil && signature.Declaration() != nil {
+		declaration = signature.Declaration()
+		if ast.IsFunctionLike(declaration) && !ast.IsFunctionTypeNode(declaration) {
+			return declaration
+		}
+	}
+
+	return nil
+}
diff --git a/internal/printer/emithost.go b/internal/printer/emithost.go
index f0a6af576..121ac712f 100644
--- a/internal/printer/emithost.go
+++ b/internal/printer/emithost.go
@@ -25,7 +25,7 @@ type EmitHost interface {
 	IsEmitBlocked(file string) bool
 	WriteFile(fileName string, text string, writeByteOrderMark bool, relatedSourceFiles []*ast.SourceFile, data *WriteFileData) error
 	GetEmitModuleFormatOfFile(file ast.HasFileName) core.ModuleKind
-	GetEmitResolver(file *ast.SourceFile) EmitResolver
+	GetEmitResolver() EmitResolver
 	GetOutputAndProjectReference(path tspath.Path) *tsoptions.OutputDtsAndProjectReference
 	IsSourceFileFromExternalLibrary(file *ast.SourceFile) bool
 }
diff --git a/internal/printer/printer.go b/internal/printer/printer.go
index 1c2dc0208..9f078e44e 100644
--- a/internal/printer/printer.go
+++ b/internal/printer/printer.go
@@ -136,6 +136,8 @@ type Printer struct {
 	inExtends                         bool // whether we are emitting the `extends` clause of a ConditionalType or InferType
 	nameGenerator                     NameGenerator
 	makeFileLevelOptimisticUniqueName func(string) string
+	commentStatePool                  core.Pool[commentState]
+	sourceMapStatePool                core.Pool[sourceMapState]
 }
 
 type detachedCommentsInfo struct {
@@ -4993,7 +4995,9 @@ func (p *Printer) emitCommentsBeforeNode(node *ast.Node) *commentState {
 		p.commentsDisabled = true
 	}
 
-	return &commentState{emitFlags, commentRange, containerPos, containerEnd, declarationListContainerEnd}
+	c := p.commentStatePool.New()
+	*c = commentState{emitFlags, commentRange, containerPos, containerEnd, declarationListContainerEnd}
+	return c
 }
 
 func (p *Printer) emitCommentsAfterNode(node *ast.Node, state *commentState) {
@@ -5046,7 +5050,7 @@ func (p *Printer) emitCommentsBeforeToken(token ast.Kind, pos int, contextNode *
 		p.decreaseIndentIf(needsIndent)
 	}
 
-	return &commentState{}, pos
+	return p.commentStatePool.New(), pos
 }
 
 func (p *Printer) emitCommentsAfterToken(token ast.Kind, pos int, contextNode *ast.Node, state *commentState) {
@@ -5486,7 +5490,9 @@ func (p *Printer) emitSourceMapsBeforeNode(node *ast.Node) *sourceMapState {
 		p.sourceMapsDisabled = true
 	}
 
-	return &sourceMapState{emitFlags, loc, false}
+	state := p.sourceMapStatePool.New()
+	*state = sourceMapState{emitFlags, loc, false}
+	return state
 }
 
 func (p *Printer) emitSourceMapsAfterNode(node *ast.Node, previousState *sourceMapState) {
@@ -5524,7 +5530,9 @@ func (p *Printer) emitSourceMapsBeforeToken(token ast.Kind, pos int, contextNode
 		}
 	}
 
-	return &sourceMapState{emitFlags, loc, hasLoc}
+	state := p.sourceMapStatePool.New()
+	*state = sourceMapState{emitFlags, loc, hasLoc}
+	return state
 }
 
 func (p *Printer) emitSourceMapsAfterToken(token ast.Kind, pos int, contextNode *ast.Node, previousState *sourceMapState) {
diff --git a/internal/project/configfileregistry.go b/internal/project/configfileregistry.go
index e17a52bdf..26b9060ba 100644
--- a/internal/project/configfileregistry.go
+++ b/internal/project/configfileregistry.go
@@ -239,7 +239,10 @@ func (c *ConfigFileRegistry) tryInvokeWildCardDirectories(fileName string, path
 	configFiles := c.ConfigFiles.ToMap()
 	for configPath, entry := range configFiles {
 		entry.mu.Lock()
-		hasSet := entry.commandLine != nil && entry.commandLine.MatchesFileName(fileName) && entry.SetPendingReload(PendingReloadFileNames)
+		hasSet := false
+		if entry.commandLine != nil && entry.pendingReload == PendingReloadNone && entry.commandLine.MatchesFileName(fileName) {
+			hasSet = entry.SetPendingReload(PendingReloadFileNames)
+		}
 		var projects map[*Project]struct{}
 		if hasSet {
 			projects = maps.Clone(entry.projects.Keys())
diff --git a/internal/project/service.go b/internal/project/service.go
index b4a5249cd..0759a7f3a 100644
--- a/internal/project/service.go
+++ b/internal/project/service.go
@@ -9,6 +9,7 @@ import (
 	"strings"
 	"sync"
 
+	"github.com/microsoft/typescript-go/internal/collections"
 	"github.com/microsoft/typescript-go/internal/core"
 	"github.com/microsoft/typescript-go/internal/ls"
 	"github.com/microsoft/typescript-go/internal/lsp/lsproto"
@@ -296,9 +297,15 @@ func (s *Service) Close() {
 }
 
 func (s *Service) OnWatchedFilesChanged(ctx context.Context, changes []*lsproto.FileEvent) error {
+	seen := collections.NewSetWithSizeHint[lsproto.FileEvent](len(changes))
+
 	s.projectsMu.RLock()
 	defer s.projectsMu.RUnlock()
 	for _, change := range changes {
+		if !seen.AddIfAbsent(*change) {
+			continue
+		}
+
 		fileName := ls.DocumentURIToFileName(change.Uri)
 		path := s.toPath(fileName)
 		if err, ok := s.configFileRegistry.onWatchedFilesChanged(path, change.Type); ok {
diff --git a/internal/transformers/declarations/transform.go b/internal/transformers/declarations/transform.go
index e9d1f39b9..639e06cbe 100644
--- a/internal/transformers/declarations/transform.go
+++ b/internal/transformers/declarations/transform.go
@@ -35,6 +35,7 @@ type DeclarationEmitHost interface {
 	GetOutputPathsFor(file *ast.SourceFile, forceDtsPaths bool) OutputPaths
 	GetResolutionModeOverride(node *ast.Node) core.ResolutionMode
 	GetEffectiveDeclarationFlags(node *ast.Node, flags ast.ModifierFlags) ast.ModifierFlags
+	GetEmitResolver() printer.EmitResolver
 }
 
 type DeclarationTransformer struct {
@@ -60,7 +61,8 @@ type DeclarationTransformer struct {
 	rawLibReferenceDirectives        []*ast.FileReference
 }
 
-func NewDeclarationTransformer(host DeclarationEmitHost, resolver printer.EmitResolver, context *printer.EmitContext, compilerOptions *core.CompilerOptions, declarationFilePath string, declarationMapPath string) *DeclarationTransformer {
+func NewDeclarationTransformer(host DeclarationEmitHost, context *printer.EmitContext, compilerOptions *core.CompilerOptions, declarationFilePath string, declarationMapPath string) *DeclarationTransformer {
+	resolver := host.GetEmitResolver()
 	state := &SymbolTrackerSharedState{isolatedDeclarations: compilerOptions.IsolatedDeclarations.IsTrue(), resolver: resolver}
 	tracker := NewSymbolTracker(host, resolver, state)
 	// TODO: Use new host GetOutputPathsFor method instead of passing in entrypoint paths (which will also better support bundled emit)
diff --git a/internal/tsoptions/parsinghelpers.go b/internal/tsoptions/parsinghelpers.go
index e13280b1a..6f4dd7466 100644
--- a/internal/tsoptions/parsinghelpers.go
+++ b/internal/tsoptions/parsinghelpers.go
@@ -2,6 +2,7 @@ package tsoptions
 
 import (
 	"reflect"
+	"strings"
 
 	"github.com/microsoft/typescript-go/internal/ast"
 	"github.com/microsoft/typescript-go/internal/collections"
@@ -488,25 +489,54 @@ func ParseTypeAcquisition(key string, value any, allOptions *core.TypeAcquisitio
 	return nil
 }
 
-// mergeCompilerOptions merges the source compiler options into the target compiler options.
-// Fields in the source options will overwrite the corresponding fields in the target options.
-func mergeCompilerOptions(targetOptions, sourceOptions *core.CompilerOptions) *core.CompilerOptions {
+// mergeCompilerOptions merges the source compiler options into the target compiler options
+// with optional awareness of explicitly set null values in the raw JSON.
+// Fields in the source options will overwrite the corresponding fields in the target options,
+// including when they are explicitly set to null in the raw configuration (if rawSource is provided).
+func mergeCompilerOptions(targetOptions, sourceOptions *core.CompilerOptions, rawSource any) *core.CompilerOptions {
 	if sourceOptions == nil {
 		return targetOptions
 	}
 
+	// Collect explicitly null field names from raw JSON
+	var explicitNullFields collections.Set[string]
+	if rawSource != nil {
+		if rawMap, ok := rawSource.(*collections.OrderedMap[string, any]); ok {
+			if compilerOptionsRaw, exists := rawMap.Get("compilerOptions"); exists {
+				if compilerOptionsMap, ok := compilerOptionsRaw.(*collections.OrderedMap[string, any]); ok {
+					for key, value := range compilerOptionsMap.Entries() {
+						if value == nil {
+							explicitNullFields.Add(key)
+						}
+					}
+				}
+			}
+		}
+	}
+
+	// Do the merge, handling explicit nulls during the normal merge
 	targetValue := reflect.ValueOf(targetOptions).Elem()
 	sourceValue := reflect.ValueOf(sourceOptions).Elem()
+	targetType := targetValue.Type()
 
 	for i := range targetValue.NumField() {
 		targetField := targetValue.Field(i)
 		sourceField := sourceValue.Field(i)
-		if sourceField.IsZero() {
-			continue
-		} else {
+
+		// Get the JSON field name for this struct field and check if it's explicitly null
+		if jsonTag := targetType.Field(i).Tag.Get("json"); jsonTag != "" {
+			if jsonFieldName, _, _ := strings.Cut(jsonTag, ","); jsonFieldName != "" && explicitNullFields.Has(jsonFieldName) {
+				targetField.SetZero()
+				continue
+			}
+		}
+
+		// Normal merge behavior: copy non-zero fields
+		if !sourceField.IsZero() {
 			targetField.Set(sourceField)
 		}
 	}
+
 	return targetOptions
 }
 
diff --git a/internal/tsoptions/tsconfigparsing.go b/internal/tsoptions/tsconfigparsing.go
index 076fba732..b527558e2 100644
--- a/internal/tsoptions/tsconfigparsing.go
+++ b/internal/tsoptions/tsconfigparsing.go
@@ -1040,7 +1040,7 @@ func parseConfig(
 					result.compileOnSave = compileOnSave
 				}
 			}
-			mergeCompilerOptions(result.options, extendedConfig.options)
+			mergeCompilerOptions(result.options, extendedConfig.options, extendsRaw)
 		}
 	}
 
@@ -1074,7 +1074,7 @@ func parseConfig(
 				sourceFile.ExtendedSourceFiles = append(sourceFile.ExtendedSourceFiles, extendedSourceFile)
 			}
 		}
-		ownConfig.options = mergeCompilerOptions(result.options, ownConfig.options)
+		ownConfig.options = mergeCompilerOptions(result.options, ownConfig.options, ownConfig.raw)
 		// ownConfig.watchOptions = ownConfig.watchOptions && result.watchOptions ?
 		//     assignWatchOptions(result, ownConfig.watchOptions) :
 		//     ownConfig.watchOptions || result.watchOptions;
@@ -1118,7 +1118,7 @@ func parseJsonConfigFileContentWorker(
 	var errors []*ast.Diagnostic
 	resolutionStackString := []string{}
 	parsedConfig, errors := parseConfig(json, sourceFile, host, basePath, configFileName, resolutionStackString, extendedConfigCache)
-	mergeCompilerOptions(parsedConfig.options, existingOptions)
+	mergeCompilerOptions(parsedConfig.options, existingOptions, nil)
 	handleOptionConfigDirTemplateSubstitution(parsedConfig.options, basePathForFileNames)
 	rawConfig := parseJsonToStringKey(parsedConfig.raw)
 	if configFileName != "" && parsedConfig.options != nil {


Test patch:
diff --git a/internal/ls/definition_test.go b/internal/ls/definition_test.go
new file mode 100644
index 000000000..a932e3eed
--- /dev/null
+++ b/internal/ls/definition_test.go
@@ -0,0 +1,74 @@
+package ls_test
+
+import (
+	"testing"
+
+	"github.com/microsoft/typescript-go/internal/bundled"
+	"github.com/microsoft/typescript-go/internal/fourslash"
+	"github.com/microsoft/typescript-go/internal/ls"
+	"github.com/microsoft/typescript-go/internal/lsp/lsproto"
+	"github.com/microsoft/typescript-go/internal/testutil/projecttestutil"
+	"gotest.tools/v3/assert"
+)
+
+func TestDefinition(t *testing.T) {
+	t.Parallel()
+	if !bundled.Embedded {
+		// Without embedding, we'd need to read all of the lib files out from disk into the MapFS.
+		// Just skip this for now.
+		t.Skip("bundled files are not embedded")
+	}
+
+	testCases := []struct {
+		title    string
+		input    string
+		expected map[string]lsproto.Definition
+	}{
+		{
+			title: "localFunction",
+			input: `
+// @filename: index.ts
+function localFunction() { }
+/*localFunction*/localFunction();`,
+			expected: map[string]lsproto.Definition{
+				"localFunction": {
+					Locations: &[]lsproto.Location{{
+						Uri:   ls.FileNameToDocumentURI("/index.ts"),
+						Range: lsproto.Range{Start: lsproto.Position{Character: 9}, End: lsproto.Position{Character: 22}},
+					}},
+				},
+			},
+		},
+	}
+
+	for _, testCase := range testCases {
+		t.Run(testCase.title, func(t *testing.T) {
+			t.Parallel()
+			runDefinitionTest(t, testCase.input, testCase.expected)
+		})
+	}
+}
+
+func runDefinitionTest(t *testing.T, input string, expected map[string]lsproto.Definition) {
+	testData := fourslash.ParseTestData(t, input, "/mainFile.ts")
+	file := testData.Files[0].FileName()
+	markerPositions := testData.MarkerPositions
+	ctx := projecttestutil.WithRequestID(t.Context())
+	languageService, done := createLanguageService(ctx, file, map[string]any{
+		file: testData.Files[0].Content,
+	})
+	defer done()
+
+	for markerName, expectedResult := range expected {
+		marker, ok := markerPositions[markerName]
+		if !ok {
+			t.Fatalf("No marker found for '%s'", markerName)
+		}
+		locations, err := languageService.ProvideDefinition(
+			ctx,
+			ls.FileNameToDocumentURI(file),
+			marker.LSPosition)
+		assert.NilError(t, err)
+		assert.DeepEqual(t, *locations, expectedResult)
+	}
+}
diff --git a/internal/transformers/tstransforms/importelision_test.go b/internal/transformers/tstransforms/importelision_test.go
index fe20f006c..f7d8b9c04 100644
--- a/internal/transformers/tstransforms/importelision_test.go
+++ b/internal/transformers/tstransforms/importelision_test.go
@@ -241,7 +241,7 @@ func TestImportElision(t *testing.T) {
 				},
 			})
 
-			emitResolver := c.GetEmitResolver(file)
+			emitResolver := c.GetEmitResolver()
 			emitResolver.MarkLinkedReferencesRecursively(file)
 
 			emitContext := printer.NewEmitContext()
diff --git a/internal/tsoptions/tsconfigparsing_test.go b/internal/tsoptions/tsconfigparsing_test.go
index 480c37ed7..618c973a4 100644
--- a/internal/tsoptions/tsconfigparsing_test.go
+++ b/internal/tsoptions/tsconfigparsing_test.go
@@ -583,6 +583,155 @@ export {}`,
 			},
 		}},
 	},
+	{
+		title:               "null overrides in extended tsconfig - array fields",
+		noSubmoduleBaseline: true,
+		input: []testConfig{{
+			jsonText: `{
+  "extends": "./tsconfig-base.json",
+  "compilerOptions": {
+    "types": null,
+    "lib": null,
+    "typeRoots": null
+  }
+}`,
+			configFileName: "tsconfig.json",
+			basePath:       "/",
+			allFileList: map[string]string{
+				"/tsconfig-base.json": `{
+  "compilerOptions": {
+    "types": ["node", "@types/jest"],
+    "lib": ["es2020", "dom"],
+    "typeRoots": ["./types", "./node_modules/@types"]
+  }
+}`,
+				"/app.ts": "",
+			},
+		}},
+	},
+	{
+		title:               "null overrides in extended tsconfig - string fields",
+		noSubmoduleBaseline: true,
+		input: []testConfig{{
+			jsonText: `{
+  "extends": "./tsconfig-base.json",
+  "compilerOptions": {
+    "outDir": null,
+    "baseUrl": null,
+    "rootDir": null
+  }
+}`,
+			configFileName: "tsconfig.json",
+			basePath:       "/",
+			allFileList: map[string]string{
+				"/tsconfig-base.json": `{
+  "compilerOptions": {
+    "outDir": "./dist",
+    "baseUrl": "./src",
+    "rootDir": "./src"
+  }
+}`,
+				"/app.ts": "",
+			},
+		}},
+	},
+	{
+		title:               "null overrides in extended tsconfig - mixed field types",
+		noSubmoduleBaseline: true,
+		input: []testConfig{{
+			jsonText: `{
+  "extends": "./tsconfig-base.json",
+  "compilerOptions": {
+    "types": null,
+    "outDir": null,
+    "strict": false,
+    "lib": ["es2022"],
+    "allowJs": null
+  }
+}`,
+			configFileName: "tsconfig.json",
+			basePath:       "/",
+			allFileList: map[string]string{
+				"/tsconfig-base.json": `{
+  "compilerOptions": {
+    "types": ["node"],
+    "lib": ["es2020", "dom"],
+    "outDir": "./dist",
+    "strict": true,
+    "allowJs": true,
+    "target": "es2020"
+  }
+}`,
+				"/app.ts": "",
+			},
+		}},
+	},
+	{
+		title:               "null overrides with multiple extends levels",
+		noSubmoduleBaseline: true,
+		input: []testConfig{{
+			jsonText: `{
+  "extends": "./tsconfig-middle.json",
+  "compilerOptions": {
+    "types": null,
+    "lib": null
+  }
+}`,
+			configFileName: "tsconfig.json",
+			basePath:       "/",
+			allFileList: map[string]string{
+				"/tsconfig-middle.json": `{
+  "extends": "./tsconfig-base.json",
+  "compilerOptions": {
+    "types": ["jest"],
+    "outDir": "./build"
+  }
+}`,
+				"/tsconfig-base.json": `{
+  "compilerOptions": {
+    "types": ["node"],
+    "lib": ["es2020"],
+    "outDir": "./dist",
+    "strict": true
+  }
+}`,
+				"/app.ts": "",
+			},
+		}},
+	},
+	{
+		title:               "null overrides in middle level of extends chain",
+		noSubmoduleBaseline: true,
+		input: []testConfig{{
+			jsonText: `{
+  "extends": "./tsconfig-middle.json",
+  "compilerOptions": {
+    "outDir": "./final"
+  }
+}`,
+			configFileName: "tsconfig.json",
+			basePath:       "/",
+			allFileList: map[string]string{
+				"/tsconfig-middle.json": `{
+  "extends": "./tsconfig-base.json",
+  "compilerOptions": {
+    "types": null,
+    "lib": null,
+    "outDir": "./middle"
+  }
+}`,
+				"/tsconfig-base.json": `{
+  "compilerOptions": {
+    "types": ["node"],
+    "lib": ["es2020"],
+    "outDir": "./base",
+    "strict": true
+  }
+}`,
+				"/app.ts": "",
+			},
+		}},
+	},
 }
 
 var tsconfigWithExtends = `{
diff --git a/testdata/baselines/reference/config/tsconfigParsing/null overrides in extended tsconfig - array fields with json api.js b/testdata/baselines/reference/config/tsconfigParsing/null overrides in extended tsconfig - array fields with json api.js
new file mode 100644
index 000000000..9a440dce3
--- /dev/null
+++ b/testdata/baselines/reference/config/tsconfigParsing/null overrides in extended tsconfig - array fields with json api.js
@@ -0,0 +1,37 @@
+Fs::
+//// [/app.ts]
+
+
+//// [/tsconfig-base.json]
+{
+  "compilerOptions": {
+    "types": ["node", "@types/jest"],
+    "lib": ["es2020", "dom"],
+    "typeRoots": ["./types", "./node_modules/@types"]
+  }
+}
+
+//// [/tsconfig.json]
+{
+  "extends": "./tsconfig-base.json",
+  "compilerOptions": {
+    "types": null,
+    "lib": null,
+    "typeRoots": null
+  }
+}
+
+
+configFileName:: tsconfig.json
+CompilerOptions::
+{
+  "configFilePath": "/tsconfig.json"
+}
+
+TypeAcquisition::
+{}
+
+FileNames::
+/app.ts
+Errors::
+
diff --git a/testdata/baselines/reference/config/tsconfigParsing/null overrides in extended tsconfig - array fields with jsonSourceFile api.js b/testdata/baselines/reference/config/tsconfigParsing/null overrides in extended tsconfig - array fields with jsonSourceFile api.js
new file mode 100644
index 000000000..9a440dce3
--- /dev/null
+++ b/testdata/baselines/reference/config/tsconfigParsing/null overrides in extended tsconfig - array fields with jsonSourceFile api.js
@@ -0,0 +1,37 @@
+Fs::
+//// [/app.ts]
+
+
+//// [/tsconfig-base.json]
+{
+  "compilerOptions": {
+    "types": ["node", "@types/jest"],
+    "lib": ["es2020", "dom"],
+    "typeRoots": ["./types", "./node_modules/@types"]
+  }
+}
+
+//// [/tsconfig.json]
+{
+  "extends": "./tsconfig-base.json",
+  "compilerOptions": {
+    "types": null,
+    "lib": null,
+    "typeRoots": null
+  }
+}
+
+
+configFileName:: tsconfig.json
+CompilerOptions::
+{
+  "configFilePath": "/tsconfig.json"
+}
+
+TypeAcquisition::
+{}
+
+FileNames::
+/app.ts
+Errors::
+
diff --git a/testdata/baselines/reference/config/tsconfigParsing/null overrides in extended tsconfig - mixed field types with json api.js b/testdata/baselines/reference/config/tsconfigParsing/null overrides in extended tsconfig - mixed field types with json api.js
new file mode 100644
index 000000000..521f2beaf
--- /dev/null
+++ b/testdata/baselines/reference/config/tsconfigParsing/null overrides in extended tsconfig - mixed field types with json api.js
@@ -0,0 +1,47 @@
+Fs::
+//// [/app.ts]
+
+
+//// [/tsconfig-base.json]
+{
+  "compilerOptions": {
+    "types": ["node"],
+    "lib": ["es2020", "dom"],
+    "outDir": "./dist",
+    "strict": true,
+    "allowJs": true,
+    "target": "es2020"
+  }
+}
+
+//// [/tsconfig.json]
+{
+  "extends": "./tsconfig-base.json",
+  "compilerOptions": {
+    "types": null,
+    "outDir": null,
+    "strict": false,
+    "lib": ["es2022"],
+    "allowJs": null
+  }
+}
+
+
+configFileName:: tsconfig.json
+CompilerOptions::
+{
+  "lib": [
+    "lib.es2022.d.ts"
+  ],
+  "strict": false,
+  "target": 7,
+  "configFilePath": "/tsconfig.json"
+}
+
+TypeAcquisition::
+{}
+
+FileNames::
+/app.ts
+Errors::
+
diff --git a/testdata/baselines/reference/config/tsconfigParsing/null overrides in extended tsconfig - mixed field types with jsonSourceFile api.js b/testdata/baselines/reference/config/tsconfigParsing/null overrides in extended tsconfig - mixed field types with jsonSourceFile api.js
new file mode 100644
index 000000000..521f2beaf
--- /dev/null
+++ b/testdata/baselines/reference/config/tsconfigParsing/null overrides in extended tsconfig - mixed field types with jsonSourceFile api.js
@@ -0,0 +1,47 @@
+Fs::
+//// [/app.ts]
+
+
+//// [/tsconfig-base.json]
+{
+  "compilerOptions": {
+    "types": ["node"],
+    "lib": ["es2020", "dom"],
+    "outDir": "./dist",
+    "strict": true,
+    "allowJs": true,
+    "target": "es2020"
+  }
+}
+
+//// [/tsconfig.json]
+{
+  "extends": "./tsconfig-base.json",
+  "compilerOptions": {
+    "types": null,
+    "outDir": null,
+    "strict": false,
+    "lib": ["es2022"],
+    "allowJs": null
+  }
+}
+
+
+configFileName:: tsconfig.json
+CompilerOptions::
+{
+  "lib": [
+    "lib.es2022.d.ts"
+  ],
+  "strict": false,
+  "target": 7,
+  "configFilePath": "/tsconfig.json"
+}
+
+TypeAcquisition::
+{}
+
+FileNames::
+/app.ts
+Errors::
+
diff --git a/testdata/baselines/reference/config/tsconfigParsing/null overrides in extended tsconfig - string fields with json api.js b/testdata/baselines/reference/config/tsconfigParsing/null overrides in extended tsconfig - string fields with json api.js
new file mode 100644
index 000000000..5e7f8ada2
--- /dev/null
+++ b/testdata/baselines/reference/config/tsconfigParsing/null overrides in extended tsconfig - string fields with json api.js
@@ -0,0 +1,37 @@
+Fs::
+//// [/app.ts]
+
+
+//// [/tsconfig-base.json]
+{
+  "compilerOptions": {
+    "outDir": "./dist",
+    "baseUrl": "./src",
+    "rootDir": "./src"
+  }
+}
+
+//// [/tsconfig.json]
+{
+  "extends": "./tsconfig-base.json",
+  "compilerOptions": {
+    "outDir": null,
+    "baseUrl": null,
+    "rootDir": null
+  }
+}
+
+
+configFileName:: tsconfig.json
+CompilerOptions::
+{
+  "configFilePath": "/tsconfig.json"
+}
+
+TypeAcquisition::
+{}
+
+FileNames::
+/app.ts
+Errors::
+
diff --git a/testdata/baselines/reference/config/tsconfigParsing/null overrides in extended tsconfig - string fields with jsonSourceFile api.js b/testdata/baselines/reference/config/tsconfigParsing/null overrides in extended tsconfig - string fields with jsonSourceFile api.js
new file mode 100644
index 000000000..5e7f8ada2
--- /dev/null
+++ b/testdata/baselines/reference/config/tsconfigParsing/null overrides in extended tsconfig - string fields with jsonSourceFile api.js
@@ -0,0 +1,37 @@
+Fs::
+//// [/app.ts]
+
+
+//// [/tsconfig-base.json]
+{
+  "compilerOptions": {
+    "outDir": "./dist",
+    "baseUrl": "./src",
+    "rootDir": "./src"
+  }
+}
+
+//// [/tsconfig.json]
+{
+  "extends": "./tsconfig-base.json",
+  "compilerOptions": {
+    "outDir": null,
+    "baseUrl": null,
+    "rootDir": null
+  }
+}
+
+
+configFileName:: tsconfig.json
+CompilerOptions::
+{
+  "configFilePath": "/tsconfig.json"
+}
+
+TypeAcquisition::
+{}
+
+FileNames::
+/app.ts
+Errors::
+
diff --git a/testdata/baselines/reference/config/tsconfigParsing/null overrides in middle level of extends chain with json api.js b/testdata/baselines/reference/config/tsconfigParsing/null overrides in middle level of extends chain with json api.js
new file mode 100644
index 000000000..2f160231d
--- /dev/null
+++ b/testdata/baselines/reference/config/tsconfigParsing/null overrides in middle level of extends chain with json api.js
@@ -0,0 +1,48 @@
+Fs::
+//// [/app.ts]
+
+
+//// [/tsconfig-base.json]
+{
+  "compilerOptions": {
+    "types": ["node"],
+    "lib": ["es2020"],
+    "outDir": "./base",
+    "strict": true
+  }
+}
+
+//// [/tsconfig-middle.json]
+{
+  "extends": "./tsconfig-base.json",
+  "compilerOptions": {
+    "types": null,
+    "lib": null,
+    "outDir": "./middle"
+  }
+}
+
+//// [/tsconfig.json]
+{
+  "extends": "./tsconfig-middle.json",
+  "compilerOptions": {
+    "outDir": "./final"
+  }
+}
+
+
+configFileName:: tsconfig.json
+CompilerOptions::
+{
+  "outDir": "/final",
+  "strict": true,
+  "configFilePath": "/tsconfig.json"
+}
+
+TypeAcquisition::
+{}
+
+FileNames::
+/app.ts
+Errors::
+
diff --git a/testdata/baselines/reference/config/tsconfigParsing/null overrides in middle level of extends chain with jsonSourceFile api.js b/testdata/baselines/reference/config/tsconfigParsing/null overrides in middle level of extends chain with jsonSourceFile api.js
new file mode 100644
index 000000000..2f160231d
--- /dev/null
+++ b/testdata/baselines/reference/config/tsconfigParsing/null overrides in middle level of extends chain with jsonSourceFile api.js
@@ -0,0 +1,48 @@
+Fs::
+//// [/app.ts]
+
+
+//// [/tsconfig-base.json]
+{
+  "compilerOptions": {
+    "types": ["node"],
+    "lib": ["es2020"],
+    "outDir": "./base",
+    "strict": true
+  }
+}
+
+//// [/tsconfig-middle.json]
+{
+  "extends": "./tsconfig-base.json",
+  "compilerOptions": {
+    "types": null,
+    "lib": null,
+    "outDir": "./middle"
+  }
+}
+
+//// [/tsconfig.json]
+{
+  "extends": "./tsconfig-middle.json",
+  "compilerOptions": {
+    "outDir": "./final"
+  }
+}
+
+
+configFileName:: tsconfig.json
+CompilerOptions::
+{
+  "outDir": "/final",
+  "strict": true,
+  "configFilePath": "/tsconfig.json"
+}
+
+TypeAcquisition::
+{}
+
+FileNames::
+/app.ts
+Errors::
+
diff --git a/testdata/baselines/reference/config/tsconfigParsing/null overrides with multiple extends levels with json api.js b/testdata/baselines/reference/config/tsconfigParsing/null overrides with multiple extends levels with json api.js
new file mode 100644
index 000000000..ada263025
--- /dev/null
+++ b/testdata/baselines/reference/config/tsconfigParsing/null overrides with multiple extends levels with json api.js
@@ -0,0 +1,48 @@
+Fs::
+//// [/app.ts]
+
+
+//// [/tsconfig-base.json]
+{
+  "compilerOptions": {
+    "types": ["node"],
+    "lib": ["es2020"],
+    "outDir": "./dist",
+    "strict": true
+  }
+}
+
+//// [/tsconfig-middle.json]
+{
+  "extends": "./tsconfig-base.json",
+  "compilerOptions": {
+    "types": ["jest"],
+    "outDir": "./build"
+  }
+}
+
+//// [/tsconfig.json]
+{
+  "extends": "./tsconfig-middle.json",
+  "compilerOptions": {
+    "types": null,
+    "lib": null
+  }
+}
+
+
+configFileName:: tsconfig.json
+CompilerOptions::
+{
+  "outDir": "/build",
+  "strict": true,
+  "configFilePath": "/tsconfig.json"
+}
+
+TypeAcquisition::
+{}
+
+FileNames::
+/app.ts
+Errors::
+
diff --git a/testdata/baselines/reference/config/tsconfigParsing/null overrides with multiple extends levels with jsonSourceFile api.js b/testdata/baselines/reference/config/tsconfigParsing/null overrides with multiple extends levels with jsonSourceFile api.js
new file mode 100644
index 000000000..ada263025
--- /dev/null
+++ b/testdata/baselines/reference/config/tsconfigParsing/null overrides with multiple extends levels with jsonSourceFile api.js
@@ -0,0 +1,48 @@
+Fs::
+//// [/app.ts]
+
+
+//// [/tsconfig-base.json]
+{
+  "compilerOptions": {
+    "types": ["node"],
+    "lib": ["es2020"],
+    "outDir": "./dist",
+    "strict": true
+  }
+}
+
+//// [/tsconfig-middle.json]
+{
+  "extends": "./tsconfig-base.json",
+  "compilerOptions": {
+    "types": ["jest"],
+    "outDir": "./build"
+  }
+}
+
+//// [/tsconfig.json]
+{
+  "extends": "./tsconfig-middle.json",
+  "compilerOptions": {
+    "types": null,
+    "lib": null
+  }
+}
+
+
+configFileName:: tsconfig.json
+CompilerOptions::
+{
+  "outDir": "/build",
+  "strict": true,
+  "configFilePath": "/tsconfig.json"
+}
+
+TypeAcquisition::
+{}
+
+FileNames::
+/app.ts
+Errors::
+