From be06031aef1394d8bd2c16fbf7817a9aa293a1f7 Mon Sep 17 00:00:00 2001
From: Jacky Zhao <j.zhao2k19@gmail.com>
Date: Thu, 24 Sep 2026 11:50:16 -0700
Subject: [PATCH 1/2] module: don't map a trailing generated-only segment

The mappings parser detects a segment that has only a generated column
by peeking at the next character. At the end of the mappings string the
peek returns an empty string, which is not a separator, so a final
generated-only segment is decoded with phantom zero deltas and inherits
the source, line, and column of the segment before it. With
--enable-source-maps, stack frames in code after the last mapped
segment are then attributed to the last mapped source.

Check for the end of the string before reading source fields.

Signed-off-by: Jacky Zhao <j.zhao2k19@gmail.com>
---
 lib/internal/source_map/source_map.js         |  3 +-
 ...st-source-map-trailing-unmapped-segment.js | 48 +++++++++++++++++++
 2 files changed, 50 insertions(+), 1 deletion(-)
 create mode 100644 test/parallel/test-source-map-trailing-unmapped-segment.js

diff --git a/lib/internal/source_map/source_map.js b/lib/internal/source_map/source_map.js
index 42e7bca3c4c5..fce0789a2e45 100644
--- a/lib/internal/source_map/source_map.js
+++ b/lib/internal/source_map/source_map.js
@@ -286,7 +286,8 @@ class SourceMap {
       }
 
       columnNumber += decodeVLQ(stringCharIterator);
-      if (isSeparator(stringCharIterator.peek())) {
+      if (!stringCharIterator.hasNext() ||
+          isSeparator(stringCharIterator.peek())) {
         ArrayPrototypePush(this.#mappings, [lineNumber, columnNumber]);
         continue;
       }
diff --git a/test/parallel/test-source-map-trailing-unmapped-segment.js b/test/parallel/test-source-map-trailing-unmapped-segment.js
new file mode 100644
index 000000000000..847a27d93113
--- /dev/null
+++ b/test/parallel/test-source-map-trailing-unmapped-segment.js
@@ -0,0 +1,48 @@
+'use strict';
+require('../common');
+const assert = require('assert');
+const { SourceMap } = require('node:module');
+
+// A segment with only a generated column marks the code after it as unmapped.
+// When it is the last segment of the mappings, it must not inherit the
+// original position of the segment before it.
+const unmapped = {
+  originalSource: undefined,
+  originalLine: undefined,
+  originalColumn: undefined,
+  name: undefined,
+};
+
+// Trailing unmapped segment on the same line as the last mapped segment.
+{
+  const sm = new SourceMap({
+    version: 3,
+    sources: ['a.js'],
+    names: [],
+    mappings: 'AAAA,K',
+  });
+
+  assert.deepStrictEqual(sm.findEntry(0, 5), {
+    generatedLine: 0,
+    generatedColumn: 5,
+    ...unmapped,
+  });
+  assert.deepStrictEqual(sm.findOrigin(1, 6), {});
+}
+
+// Trailing unmapped segment on a later line.
+{
+  const sm = new SourceMap({
+    version: 3,
+    sources: ['a.js'],
+    names: [],
+    mappings: 'AAAA;A',
+  });
+
+  assert.deepStrictEqual(sm.findEntry(1, 3), {
+    generatedLine: 1,
+    generatedColumn: 0,
+    ...unmapped,
+  });
+  assert.deepStrictEqual(sm.findOrigin(2, 4), {});
+}

From 9d71a74b530412e35e31d94e75b4e66559dece03 Mon Sep 17 00:00:00 2001
From: Jacky Zhao <j.zhao2k19@gmail.com>
Date: Fri, 9 Oct 2026 11:08:23 -0700
Subject: [PATCH 2/2] fixup! module: don't map a trailing generated-only
 segment

Check hasNext() before reading the optional name field, and add a test
for a segment that has no name.

Signed-off-by: Jacky Zhao <j.zhao2k19@gmail.com>
Assisted-by: Claude Code
---
 lib/internal/source_map/source_map.js         |  3 ++-
 ...st-source-map-trailing-unmapped-segment.js | 19 +++++++++++++++++++
 2 files changed, 21 insertions(+), 1 deletion(-)

diff --git a/lib/internal/source_map/source_map.js b/lib/internal/source_map/source_map.js
index fce0789a2e45..2bb23b00c167 100644
--- a/lib/internal/source_map/source_map.js
+++ b/lib/internal/source_map/source_map.js
@@ -301,7 +301,8 @@ 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-unmapped-segment.js b/test/parallel/test-source-map-trailing-unmapped-segment.js
index 847a27d93113..d5c7243ad999 100644
--- a/test/parallel/test-source-map-trailing-unmapped-segment.js
+++ b/test/parallel/test-source-map-trailing-unmapped-segment.js
@@ -46,3 +46,22 @@ const unmapped = {
   });
   assert.deepStrictEqual(sm.findOrigin(2, 4), {});
 }
+
+// A segment without a name field must not pick up a name from the names array.
+{
+  const sm = new SourceMap({
+    version: 3,
+    sources: ['a.js'],
+    names: ['should-not-be-used'],
+    mappings: 'AAAA',
+  });
+
+  assert.deepStrictEqual(sm.findEntry(0, 0), {
+    generatedLine: 0,
+    generatedColumn: 0,
+    originalSource: 'a.js',
+    originalLine: 0,
+    originalColumn: 0,
+    name: undefined,
+  });
+}