From 7f5f92773de79a620257168f0c4c8178ae4d5a89 Mon Sep 17 00:00:00 2001
From: Tim Fish <tim@timfish.uk>
Date: Wed, 17 Jun 2026 17:17:44 +0100
Subject: [PATCH 1/4] fix: Pass through sourcemaps

---
 lib/transformer.js   |  5 ++---
 tests/tests.test.mjs | 17 +++++++++++++++++
 2 files changed, 19 insertions(+), 3 deletions(-)

diff --git a/lib/transformer.js b/lib/transformer.js
index 663e3cb..1e40320 100644
--- a/lib/transformer.js
+++ b/lib/transformer.js
@@ -134,12 +134,11 @@ class Transformer {
     }
 
     if (ast) {
-      SourceMapConsumer ??= require('source-map').SourceMapConsumer
       SourceMapGenerator ??= require('source-map').SourceMapGenerator
 
       const file = `${this.#moduleName}/${this.#filePath}`
-      const sourceMapInput = sourcemap ? new SourceMapConsumer(sourcemap) : { file }
-      const sourceMap = new SourceMapGenerator(sourceMapInput)
+      const sourceMap = new SourceMapGenerator({ file })
+
       const code = generate(ast, { sourceMap })
       const map = sourceMap.toString()
 
diff --git a/tests/tests.test.mjs b/tests/tests.test.mjs
index 225bcb8..e2eccdc 100644
--- a/tests/tests.test.mjs
+++ b/tests/tests.test.mjs
@@ -516,6 +516,23 @@ describe('source_map', () => {
     assert.equal(original.line, originalReturnLine)
     assert.equal(original.column, originalReturnColumn)
   })
+
+  test('produces a valid sourcemap when an inputSourceMap is provided', () => {
+    const code = 'export class Undici { async fetch (url) { return 42; } }'
+    const inputSourceMap = { version: 3, sources: ['input.js'], mappings: 'AAAA', names: [] }
+
+    const instrumentor = create([
+      {
+        channelName: 'Undici:fetch',
+        module: { name: TEST_MODULE_NAME, versionRange: '>=0.0.1', filePath: TEST_MODULE_PATH },
+        functionQuery: { className: 'Undici', methodName: 'fetch', kind: 'Async' },
+      },
+    ])
+    const transformer = instrumentor.getTransformer(TEST_MODULE_NAME, TEST_MODULE_VERSION, TEST_MODULE_PATH)
+    const { map } = transformer.transform(code, 'esm', inputSourceMap)
+
+    assert.equal(JSON.parse(map).file, `${TEST_MODULE_NAME}/${TEST_MODULE_PATH}`)
+  })
 })
 
 describe('wrap_promise_non_promise', () => {

From 6dd009c1da72df62e4bb930eb42a1c91d0c7c999 Mon Sep 17 00:00:00 2001
From: Tim Fish <tim@timfish.uk>
Date: Wed, 17 Jun 2026 17:36:14 +0100
Subject: [PATCH 2/4] Fix lint

---
 lib/transformer.js | 1 -
 1 file changed, 1 deletion(-)

diff --git a/lib/transformer.js b/lib/transformer.js
index 1e40320..56d6676 100644
--- a/lib/transformer.js
+++ b/lib/transformer.js
@@ -5,7 +5,6 @@ const { parse } = require('meriyah')
 const { generate } = require('astring')
 const transforms = require('./transforms')
 
-let SourceMapConsumer
 let SourceMapGenerator
 
 /**

From 6f69d9f968d826116562b4fd549e8d6ca59c59d9 Mon Sep 17 00:00:00 2001
From: Tim Fish <tim@timfish.uk>
Date: Wed, 17 Jun 2026 18:02:42 +0100
Subject: [PATCH 3/4] Fix

---
 lib/transformer.js | 15 ++++++++++++---
 1 file changed, 12 insertions(+), 3 deletions(-)

diff --git a/lib/transformer.js b/lib/transformer.js
index 56d6676..76d6c4d 100644
--- a/lib/transformer.js
+++ b/lib/transformer.js
@@ -5,6 +5,7 @@ const { parse } = require('meriyah')
 const { generate } = require('astring')
 const transforms = require('./transforms')
 
+let SourceMapConsumer
 let SourceMapGenerator
 
 /**
@@ -133,13 +134,21 @@ class Transformer {
     }
 
     if (ast) {
+      SourceMapConsumer ??= require('source-map').SourceMapConsumer
       SourceMapGenerator ??= require('source-map').SourceMapGenerator
 
       const file = `${this.#moduleName}/${this.#filePath}`
-      const sourceMap = new SourceMapGenerator({ file })
+      let generator
+      if (sourcemap) {
+        const consumer = new SourceMapConsumer(sourcemap)
+        consumer.file = file
+        generator = SourceMapGenerator.fromSourceMap(consumer)
+      } else {
+        generator = new SourceMapGenerator({ file })
+      }
 
-      const code = generate(ast, { sourceMap })
-      const map = sourceMap.toString()
+      const code = generate(ast, { sourceMap: generator })
+      const map = generator.toString()
 
       return { code, map }
     }

From 865f38924f2c24da92040fb1797aa6816a5202f5 Mon Sep 17 00:00:00 2001
From: Tim Fish <tim@timfish.uk>
Date: Wed, 17 Jun 2026 18:06:40 +0100
Subject: [PATCH 4/4] Improve types

---
 index.d.ts | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/index.d.ts b/index.d.ts
index 7d2ec98..380e404 100644
--- a/index.d.ts
+++ b/index.d.ts
@@ -123,5 +123,5 @@ export class Transformer {
    * # Errors
    * Returns an error if the transformation fails to find injection points.
    */
-  transform(code: string | Buffer, moduleType: ModuleType, sourcemap?: string | null): TransformOutput;
+  transform(code: string | Buffer, moduleType: ModuleType, sourcemap?: string | object | null): TransformOutput;
 }