Reuse existing var

src/compiler/binder.ts

@@ -1418,7 +1418,7 @@ namespace ts {
             else {
                 const parent = node.parent as SourceFile;
 
-                if (!isExternalModule(<SourceFile>node.parent)) {
+                if (!isExternalModule(parent)) {
                     file.bindDiagnostics.push(createDiagnosticForNode(node, Diagnostics.Global_module_exports_may_only_appear_in_module_files));
                     return;
                 }