From 8a24fe75c94d4dc2763437bb15b98eb0d08702e4 Mon Sep 17 00:00:00 2001
From: Jake Bailey <5341706+jakebailey@users.noreply.github.com>
Date: Wed, 10 Aug 2022 16:38:38 -0400
Subject: [PATCH] Fix up code so we don't crash when TS itself is emitted with
 let/const (#50151)

---
 src/compiler/checker.ts  | 6 ++++--
 src/services/services.ts | 6 +++++-
 2 files changed, 9 insertions(+), 3 deletions(-)

diff --git a/src/compiler/checker.ts b/src/compiler/checker.ts
index ff3499f5461..d3f4581ee0c 100644
--- a/src/compiler/checker.ts
+++ b/src/compiler/checker.ts
@@ -27075,9 +27075,11 @@ namespace ts {
                 case AssignmentDeclarationKind.ExportsProperty:
                 case AssignmentDeclarationKind.Prototype:
                 case AssignmentDeclarationKind.PrototypeProperty:
-                    let valueDeclaration = binaryExpression.left.symbol?.valueDeclaration;
-                    // falls through
                 case AssignmentDeclarationKind.ModuleExports:
+                    let valueDeclaration: Declaration | undefined;
+                    if (kind !== AssignmentDeclarationKind.ModuleExports) {
+                        valueDeclaration = binaryExpression.left.symbol?.valueDeclaration;
+                    }
                     valueDeclaration ||= binaryExpression.symbol?.valueDeclaration;
                     const annotated = valueDeclaration && getEffectiveTypeAnnotationNode(valueDeclaration);
                     return annotated ? getTypeFromTypeNode(annotated) : undefined;
diff --git a/src/services/services.ts b/src/services/services.ts
index ecd7eae9530..15540e28acf 100644
--- a/src/services/services.ts
+++ b/src/services/services.ts
@@ -1363,6 +1363,11 @@ namespace ts {
                 onUnRecoverableConfigFileDiagnostic: noop,
             };
 
+            // The call to isProgramUptoDate below may refer back to documentRegistryBucketKey;
+            // calculate this early so it's not undefined if downleveled to a var (or, if emitted
+            // as a const variable without downleveling, doesn't crash).
+            const documentRegistryBucketKey = documentRegistry.getKeyForCompilationSettings(newSettings);
+
             // If the program is already up-to-date, we can reuse it
             if (isProgramUptoDate(program, rootFileNames, newSettings, (_path, fileName) => host.getScriptVersion(fileName), fileName => compilerHost!.fileExists(fileName), hasInvalidatedResolution, hasChangedAutomaticTypeDirectiveNames, getParsedCommandLine, projectReferences)) {
                 return;
@@ -1374,7 +1379,6 @@ namespace ts {
             // the program points to old source files that have been invalidated because of
             // incremental parsing.
 
-            const documentRegistryBucketKey = documentRegistry.getKeyForCompilationSettings(newSettings);
             const options: CreateProgramOptions = {
                 rootNames: rootFileNames,
                 options: newSettings,