From 69543f48458d8439e3da9e737dbce1b8060cfb12 Mon Sep 17 00:00:00 2001
From: Dusan Stanojevic <dusan.stanojevic.cs@gmail.com>
Date: Mon, 8 Jun 2026 13:41:02 -0500
Subject: [PATCH] module: don't read a phantom name on the last mapping segment

SourceMap#parseMap reads the optional name VLQ guarded only by
!isSeparator(peek()). At the end of the mappings string peek() returns
'', which is not a separator, so the parser decodes a phantom VLQ (0),
keeps the previous nameIndex, and assigns names[nameIndex] to a segment
that has no name field. SourceMap.findEntry() then returns a stale name,
which surfaces as the wrong function name on the bottom global frame of
--enable-source-maps stack traces.

Guard the name read with hasNext() so the phantom field is not read at
end of string. A trailing segment that genuinely carries a name still
has characters left to read, so it is unaffected.

Refs: https://github.com/nodejs/node/issues/63795
Signed-off-by: Dusan Stanojevic <dusan.stanojevic.cs@gmail.com>
---
 lib/internal/source_map/source_map.js         |  3 +-
 .../parallel/test-source-map-trailing-name.js | 34 +++++++++++++++++++
 2 files changed, 36 insertions(+), 1 deletion(-)
 create mode 100644 test/parallel/test-source-map-trailing-name.js

diff --git a/lib/internal/source_map/source_map.js b/lib/internal/source_map/source_map.js
index 42e7bca3c4c5c0..cda2dabc9ab7a5 100644
--- a/lib/internal/source_map/source_map.js
+++ b/lib/internal/source_map/source_map.js
@@ -300,7 +300,8 @@ class SourceMap {
       sourceColumnNumber += decodeVLQ(stringCharIterator);
 
       let name;
-      if (!isSeparator(stringCharIterator.peek())) {
+      if (stringCharIterator.hasNext() &&
+          !isSeparator(stringCharIterator.peek())) {
         nameIndex += decodeVLQ(stringCharIterator);
         name = map.names?.[nameIndex];
       }
diff --git a/test/parallel/test-source-map-trailing-name.js b/test/parallel/test-source-map-trailing-name.js
new file mode 100644
index 00000000000000..55867b811b0010
--- /dev/null
+++ b/test/parallel/test-source-map-trailing-name.js
@@ -0,0 +1,34 @@
+'use strict';
+require('../common');
+const assert = require('assert');
+const { SourceMap } = require('node:module');
+
+// Refs: https://github.com/nodejs/node/issues/63795
+
+// A mapping whose final segment has 4 fields (no name index) must not
+// inherit the previous segment's name.
+{
+  const sm = new SourceMap({
+    version: 3,
+    sources: ['a.js'],
+    names: ['foo'],
+    mappings: 'AAAAA,CAAC',
+  });
+
+  assert.strictEqual(sm.findEntry(0, 0).name, 'foo');
+  assert.strictEqual(sm.findEntry(0, 1).name, undefined);
+}
+
+// A mapping whose final segment legitimately carries a name index must still
+// resolve it: the end-of-string guard must not suppress a real trailing name.
+{
+  const sm = new SourceMap({
+    version: 3,
+    sources: ['a.js'],
+    names: ['foo'],
+    mappings: 'AAAA,CAAAA',
+  });
+
+  assert.strictEqual(sm.findEntry(0, 0).name, undefined);
+  assert.strictEqual(sm.findEntry(0, 1).name, 'foo');
+}