From 607bf8f60be3b50820176cc0efe53aec2ac633eb Mon Sep 17 00:00:00 2001
From: Bronley Plumb <bronley@gmail.com>
Date: Fri, 29 May 2026 14:01:32 -0400
Subject: [PATCH] Guard transpileComments against undefined trivia tokens

The lexer emits a token's `leadingTrivia` as `undefined` (rather than
`[]`) when no whitespace, newlines, or comments precede it. An
`@annotation` whose `at` token has no preceding trivia therefore has
`leadingTrivia === undefined`, which `AnnotationExpression.transpile`
forwards directly to `state.transpileComments(this.leadingTrivia)`,
crashing with `TypeError: Cannot read properties of undefined
(reading 'filter')`.

Match the existing guard in `transpileLeadingComments` and the
nullish-tolerant helpers in `util.ts` (`hasLeadingComments`,
`getLeadingComments`) by returning early when `tokens` is nullish.
---
 src/files/BrsFile.spec.ts    | 15 +++++++++++++++
 src/parser/TranspileState.ts |  3 +++
 2 files changed, 18 insertions(+)

diff --git a/src/files/BrsFile.spec.ts b/src/files/BrsFile.spec.ts
index 238589590..b5d81f5a9 100644
--- a/src/files/BrsFile.spec.ts
+++ b/src/files/BrsFile.spec.ts
@@ -3628,6 +3628,21 @@ describe('BrsFile', () => {
             `, undefined, 'source/main.bs');
         });
 
+        it('does not crash when annotation has undefined leadingTrivia', async () => {
+            //the lexer emits leadingTrivia as `undefined` (rather than `[]`) when a token has no preceding trivia,
+            //so an `@annotation` whose `at` token has no preceding whitespace/comments/newlines produces
+            //`at.leadingTrivia === undefined`. `AnnotationExpression.transpile` forwards that directly to
+            //`state.transpileComments`, which must tolerate a nullish input rather than crashing on `.filter`
+            //of undefined. Feed source that starts at column 0 with no leading newline to ensure no trivia
+            //is attached to the `@` token.
+            program.setFile('source/main.bs', `@annotation\nsub main()\n    print "main"\nend sub`);
+            program.validate();
+            expectZeroDiagnostics(program);
+            //this should not throw
+            const result = await program.getTranspiledFileContents('source/main.bs');
+            expect(result.code).to.include('sub main()');
+        });
+
         it('includes annotation comments for class', async () => {
             await testTranspile(`
                 'comment1
diff --git a/src/parser/TranspileState.ts b/src/parser/TranspileState.ts
index fced4e45d..97d31b6c9 100644
--- a/src/parser/TranspileState.ts
+++ b/src/parser/TranspileState.ts
@@ -141,6 +141,9 @@ export class TranspileState {
 
     public transpileComments(tokens: TranspileToken[], prepNextLine = false): Array<string | SourceNode> {
         const leadingCommentsSourceNodes = [];
+        if (!tokens) {
+            return leadingCommentsSourceNodes;
+        }
         const justComments = tokens.filter(t => t.kind === TokenKind.Comment || t.kind === TokenKind.Newline);
         let newLinesSinceComment = 0;