diff --git a/src/compiler/core.ts b/src/compiler/core.ts
index e0e70c7294364..933c599c41a71 100644
--- a/src/compiler/core.ts
+++ b/src/compiler/core.ts
@@ -1381,6 +1381,17 @@ namespace ts {
         return keys;
     }
 
+    export function getAllKeys(obj: object): string[] {
+        const result: string[] = [];
+        do {
+            const names = Object.getOwnPropertyNames(obj);
+            for (const name of names) {
+                pushIfUnique(result, name);
+            }
+        } while (obj = Object.getPrototypeOf(obj));
+        return result;
+    }
+
     export function getOwnValues<T>(sparseArray: T[]): T[] {
         const values: T[] = [];
         for (const key in sparseArray) {
diff --git a/src/compiler/program.ts b/src/compiler/program.ts
index 6cbc9258c0cf4..a7cb013c8eebc 100644
--- a/src/compiler/program.ts
+++ b/src/compiler/program.ts
@@ -814,7 +814,10 @@ namespace ts {
         let mapFromFileToProjectReferenceRedirects: Map<Path> | undefined;
 
         const shouldCreateNewSourceFile = shouldProgramCreateNewSourceFiles(oldProgram, options);
-        const structuralIsReused = tryReuseStructureFromOldProgram();
+        // We set `structuralIsReused` to `undefined` because `tryReuseStructureFromOldProgram` calls `tryReuseStructureFromOldProgram` which checks
+        // `structuralIsReused`, which would be a TDZ violation if it was not set in advance to `undefined`.
+        let structuralIsReused: StructureIsReused | undefined;
+        structuralIsReused = tryReuseStructureFromOldProgram();
         if (structuralIsReused !== StructureIsReused.Completely) {
             processingDefaultLibFiles = [];
             processingOtherFiles = [];
diff --git a/src/compiler/transformers/declarations.ts b/src/compiler/transformers/declarations.ts
index bb939f7d4f1f1..6dfc909b93abd 100644
--- a/src/compiler/transformers/declarations.ts
+++ b/src/compiler/transformers/declarations.ts
@@ -83,6 +83,7 @@ namespace ts {
         let currentSourceFile: SourceFile;
         let refs: Map<SourceFile>;
         let libs: Map<boolean>;
+        let emittedImports: readonly AnyImportSyntax[] | undefined; // must be declared in container so it can be `undefined` while transformer's first pass
         const resolver = context.getEmitResolver();
         const options = context.getCompilerOptions();
         const newLine = getNewLineCharacter(options);
@@ -279,7 +280,7 @@ namespace ts {
             const statements = visitNodes(node.statements, visitDeclarationStatements);
             let combinedStatements = setTextRange(createNodeArray(transformAndReplaceLatePaintedStatements(statements)), node.statements);
             refs.forEach(referenceVisitor);
-            const emittedImports = filter(combinedStatements, isAnyImportSyntax);
+            emittedImports = filter(combinedStatements, isAnyImportSyntax);
             if (isExternalModule(node) && (!resultHasExternalModuleIndicator || (needsScopeFixMarker && !resultHasScopeMarker))) {
                 combinedStatements = setTextRange(createNodeArray([...combinedStatements, createEmptyExports()]), combinedStatements);
             }
@@ -736,6 +737,12 @@ namespace ts {
             }
             const oldDiag = getSymbolAccessibilityDiagnostic;
 
+            // Setup diagnostic-related flags before first potential `cleanup` call, otherwise
+            // We'd see a TDZ violation at runtime
+            const canProduceDiagnostic = canProduceDiagnostics(input);
+            const oldWithinObjectLiteralType = suppressNewDiagnosticContexts;
+            let shouldEnterSuppressNewDiagnosticsContextContext = ((input.kind === SyntaxKind.TypeLiteral || input.kind === SyntaxKind.MappedType) && input.parent.kind !== SyntaxKind.TypeAliasDeclaration);
+
             // Emit methods which are private as properties with no type information
             if (isMethodDeclaration(input) || isMethodSignature(input)) {
                 if (hasModifier(input, ModifierFlags.Private)) {
@@ -744,8 +751,7 @@ namespace ts {
                 }
             }
 
-            const canProdiceDiagnostic = canProduceDiagnostics(input);
-            if (canProdiceDiagnostic && !suppressNewDiagnosticContexts) {
+            if (canProduceDiagnostic && !suppressNewDiagnosticContexts) {
                 getSymbolAccessibilityDiagnostic = createGetSymbolAccessibilityDiagnosticForNode(input as DeclarationDiagnosticProducing);
             }
 
@@ -753,8 +759,6 @@ namespace ts {
                 checkEntityNameVisibility(input.exprName, enclosingDeclaration);
             }
 
-            const oldWithinObjectLiteralType = suppressNewDiagnosticContexts;
-            let shouldEnterSuppressNewDiagnosticsContextContext = ((input.kind === SyntaxKind.TypeLiteral || input.kind === SyntaxKind.MappedType) && input.parent.kind !== SyntaxKind.TypeAliasDeclaration);
             if (shouldEnterSuppressNewDiagnosticsContextContext) {
                 // We stop making new diagnostic contexts within object literal types. Unless it's an object type on the RHS of a type alias declaration. Then we do.
                 suppressNewDiagnosticContexts = true;
@@ -909,13 +913,13 @@ namespace ts {
             return cleanup(visitEachChild(input, visitDeclarationSubtree, context));
 
             function cleanup<T extends Node>(returnValue: T | undefined): T | undefined {
-                if (returnValue && canProdiceDiagnostic && hasDynamicName(input as Declaration)) {
+                if (returnValue && canProduceDiagnostic && hasDynamicName(input as Declaration)) {
                     checkName(input as DeclarationDiagnosticProducing);
                 }
                 if (isEnclosingDeclaration(input)) {
                     enclosingDeclaration = previousEnclosingDeclaration;
                 }
-                if (canProdiceDiagnostic && !suppressNewDiagnosticContexts) {
+                if (canProduceDiagnostic && !suppressNewDiagnosticContexts) {
                     getSymbolAccessibilityDiagnostic = oldDiag;
                 }
                 if (shouldEnterSuppressNewDiagnosticsContextContext) {
diff --git a/src/harness/fourslash.ts b/src/harness/fourslash.ts
index f12506be2fb27..8f1a5ae15280d 100644
--- a/src/harness/fourslash.ts
+++ b/src/harness/fourslash.ts
@@ -344,7 +344,8 @@ namespace FourSlash {
                     "getDocumentHighlights",
                 ];
                 const proxy = {} as ts.LanguageService;
-                for (const k in ls) {
+                const keys = ts.getAllKeys(ls);
+                for (const k of keys) {
                     const key = k as keyof typeof ls;
                     if (cacheableMembers.indexOf(key) === -1) {
                         proxy[key] = (...args: any[]) => (ls[key] as Function)(...args);