From e99954cde6b161a945f1a04200b476443c59fa1b Mon Sep 17 00:00:00 2001
From: James Sumners <jsumners@newrelic.com>
Date: Tue, 19 Aug 2025 15:14:51 -0400
Subject: [PATCH 01/10] Added support for traceCallback

---
 src/function_query.rs      |  8 +++++-
 src/instrumentation.rs     | 56 +++++++++++++++++++++++++-------------
 tests/Readme.md            | 47 ++++++++++++++++++++++++++++++++
 tests/callback_cjs/mod.js  | 12 ++++++++
 tests/callback_cjs/mod.rs  | 24 ++++++++++++++++
 tests/callback_cjs/test.js | 46 +++++++++++++++++++++++++++++++
 tests/common/mod.rs        | 30 +++++++++++++++++++-
 tests/instrumentor_test.rs |  1 +
 8 files changed, 203 insertions(+), 21 deletions(-)
 create mode 100644 tests/Readme.md
 create mode 100644 tests/callback_cjs/mod.js
 create mode 100644 tests/callback_cjs/mod.rs
 create mode 100644 tests/callback_cjs/test.js

diff --git a/src/function_query.rs b/src/function_query.rs
index e8e7a4a..cef3599 100644
--- a/src/function_query.rs
+++ b/src/function_query.rs
@@ -17,6 +17,7 @@ pub(crate) enum FunctionType {
 pub enum FunctionKind {
     Sync,
     Async,
+    Callback
 }
 
 impl FunctionKind {
@@ -24,12 +25,17 @@ impl FunctionKind {
     pub fn is_async(&self) -> bool {
         matches!(self, FunctionKind::Async)
     }
+    
+    pub fn is_callback(&self) -> bool {
+        matches!(self, FunctionKind::Callback)
+    }
 
     #[must_use]
     pub fn tracing_operator(&self) -> &'static str {
         match self {
             FunctionKind::Sync => "traceSync",
             FunctionKind::Async => "tracePromise",
+            FunctionKind::Callback => "traceCallback"
         }
     }
 }
@@ -43,7 +49,7 @@ impl FunctionKind {
 #[derive(Debug, Clone)]
 pub enum FunctionQuery {
     // The order here matters because this enum is untagged, serde will try
-    // choose the first variant that matches the data.
+    // to choose the first variant that matches the data.
     ClassMethod {
         class_name: String,
         method_name: String,
diff --git a/src/instrumentation.rs b/src/instrumentation.rs
index 984767d..3218779 100644
--- a/src/instrumentation.rs
+++ b/src/instrumentation.rs
@@ -119,7 +119,8 @@ impl Instrumentation {
             ctxt: SyntaxContext::empty(),
             stmts: vec![
                 quote!("const __apm$wrapped = $wrapped;" as Stmt, wrapped: Expr = wrapped_fn.into()),
-                quote!("return __apm$wrapped.apply(null, __apm$original_args);" as Stmt),
+                // TODO: instead of a static `this` we should be able to declare the value
+                quote!("return __apm$wrapped.apply(this, __apm$original_args);" as Stmt),
             ],
         };
 
@@ -127,11 +128,11 @@ impl Instrumentation {
 
         let id_name = self.config.get_identifier_name();
         let ch_ident = ident!(format!("tr_ch_apm${}", &id_name));
-        let trace_ident = ident!(format!(
-            "tr_ch_apm${}.{}",
+        let trace_statement = construct_trace_statement(
+            &self.config,
             &id_name,
-            self.config.function_query.kind().tracing_operator()
-        ));
+            (&self.module_version).clone().unwrap_or(String::new())
+        );
 
         body.stmts = vec![
             quote!("const __apm$original_args = arguments" as Stmt),
@@ -140,18 +141,7 @@ impl Instrumentation {
                 "if (!$ch.hasSubscribers) return __apm$traced();" as Stmt,
                 ch = ch_ident
             ),
-            match &self.module_version {
-                Some(version) => quote!(
-                    "return $trace(__apm$traced, { arguments, self: this, moduleVersion: $version } );"
-                        as Stmt,
-                    trace = trace_ident,
-                    version: Expr = version.as_str().into(),
-                ),
-                None => quote!(
-                    "return $trace(__apm$traced, { arguments, self: this } );" as Stmt,
-                    trace = trace_ident,
-                ),
-            },
+            trace_statement
         ];
 
         self.has_injected = true;
@@ -235,7 +225,7 @@ impl Instrumentation {
     }
 
     // The rest of these functions are from `VisitMut`, except they return a boolean to indicate
-    // whether recusrsing through the tree is necessary, rather than calling
+    // whether recursing through the tree is necessary, rather than calling
     // `visit_mut_children_with`.
 
     pub fn visit_mut_module(&mut self, node: &mut Module) -> bool {
@@ -356,7 +346,7 @@ impl Instrumentation {
 
     pub fn visit_mut_assign_expr(&mut self, node: &mut AssignExpr) -> bool {
         // TODO(bengl) This is by far the hardest bit. We're trying to infer a name for this
-        // function expresion using the surrounding code, but it's not always possible, and even
+        // function expression using the surrounding code, but it's not always possible, and even
         // where it is, there are so many ways to give a function expression a "name", that the
         // code paths here can get pretty hairy. Right now this is only covering some basic cases.
         // The following cases are missing:
@@ -398,3 +388,31 @@ pub fn get_script_start_index(script: &Script) -> usize {
     }
     0
 }
+
+/// Builds a tracing channel trace invocation. For example, the result may
+/// look like:
+///
+/// ```js
+/// return tr_ch_amp$myChannel.traceSync(__apm$traced, { arguments, self: this, moduleVersion: "1.0.0" })
+/// ```
+fn construct_trace_statement(config: &InstrumentationConfig, channel_name: &String, mod_version: String) -> Stmt {
+    let mut ctx = "{ arguments, self: this }".to_string();
+    if mod_version.len() > 0 {
+        ctx = ["{ arguments, self: this, moduleVersion: \"", mod_version.as_str(), "\" }"].join("");
+    }
+
+    let operator = ["tr_ch_apm$", channel_name, ".", config.function_query.kind().tracing_operator()].join("");
+    let stmt_str;
+    if config.function_query.kind().is_callback() == true {
+        // TODO: read callback position from configuration
+        // TODO: figure out how we can pass a `this` argument
+        stmt_str = ["return ", &*operator, "(__apm$traced, -1, ", ctx.as_str(), ", this, ...arguments)"].join("");
+    } else {
+        stmt_str = ["return ", &*operator, "(__apm$traced, ", ctx.as_str(), ")"].join("");
+    }
+
+    return quote!(
+        "$stmt" as Stmt,
+        stmt = Ident::new(Atom::from(stmt_str), Span::default(), SyntaxContext::empty())
+    )
+}
\ No newline at end of file
diff --git a/tests/Readme.md b/tests/Readme.md
new file mode 100644
index 0000000..b46d9cb
--- /dev/null
+++ b/tests/Readme.md
@@ -0,0 +1,47 @@
+# Unit Tests
+
+Unit tests in this project follow a specific structure:
+
+1. A directory named the same as the unit test. For example,
+the `index_cjs` test will be within the `index_cjs/` directory.
+2. A `mod.rs` file that contains the unit test.
+3. A `mod.{js,mjs}` file that contains a JavaScript script to apply
+the test against, i.e. this replicates a potential module from
+npmjs.com.
+4. A `test.js` file that exercises the patched code.
+5. A new `mod` line in the [`instrumentor_test.rs`](./instrumentor_test.rs)
+file.
+
+To run a specific test from the command line:
+
+```sh
+# To run the index_cjs test only:
+cargo test index_cjs
+```
+
+Each unit test should utilize the `transpile_and_test` function
+exported by [`common/mod.rs`](./common/mod.rs). This function:
+
+1. Reads in the unit test file to get the base path to the test.
+2. Reads in the JavaScript module file.
+3. Transforms the read-in JavaScript according to the defined configuration.
+4. Generates a new `instrumented.js` file within the test directory. This
+file contains the `mod.{js,mjs}` code patched by our tool.
+5. Runs `node test.js` within the test directory. The result of this test
+should be a `0` exit code for the process.
+   
+## Running In A Debugger
+
+### RustRover
+
+To run a specific test through the [RustRover](https://www.jetbrains.com/rust/)
+debugger:
+
+1. Create a new run configuration of type "Cargo".
+2. For the "command", enter (e.g. for the `index_cjs` test):
+
+        test --package orchestrion-js --test instrumentor_test index_cjs::index_cjs -- --exact
+3. Set breakpoints and run the profile.
+
+For a shortcut, open the desired `mod.rs` test file and click the "run"
+button in the gutter.
\ No newline at end of file
diff --git a/tests/callback_cjs/mod.js b/tests/callback_cjs/mod.js
new file mode 100644
index 0000000..171f50d
--- /dev/null
+++ b/tests/callback_cjs/mod.js
@@ -0,0 +1,12 @@
+'use strict'
+
+module.exports = Foo
+
+function Foo() {}
+
+Foo.prototype.doWork = function doWork(input, callback) {
+  if (input === 'fail') {
+    return callback(Error('boom'))
+  }
+  return callback(null, 'done')
+}
\ No newline at end of file
diff --git a/tests/callback_cjs/mod.rs b/tests/callback_cjs/mod.rs
new file mode 100644
index 0000000..700d896
--- /dev/null
+++ b/tests/callback_cjs/mod.rs
@@ -0,0 +1,24 @@
+use crate::common::*;
+use orchestrion_js::*;
+
+#[test]
+fn callback_cjs() {
+    transpile_and_test_with_imports(
+        file!(),
+        false,
+        Config::new_single(InstrumentationConfig::new(
+            "foo_ch",
+            ModuleMatcher::new("foo", ">=1.0.0", "index.js").unwrap(),
+            FunctionQuery::FunctionExpression {
+                expression_name: "doWork".to_string(),
+                kind: FunctionKind::Callback,
+                index: 0
+            }
+        )),
+        &[PackageImport {
+            module_name: "foo".to_string(),
+            module_version: "1.1.1".to_string(),
+            file: "index.js".to_string()
+        }],
+    )
+}
\ No newline at end of file
diff --git a/tests/callback_cjs/test.js b/tests/callback_cjs/test.js
new file mode 100644
index 0000000..90278d3
--- /dev/null
+++ b/tests/callback_cjs/test.js
@@ -0,0 +1,46 @@
+'use strict'
+
+const assert = require('node:assert')
+const { getContext } = require('../common/preamble.js');
+const context = getContext('orchestrion:foo:foo_ch');
+const Foo = require('./instrumented.js')
+
+const start = process.hrtime.bigint()
+const timer1 = setInterval(timeout, 1_000)
+const timer2 = setInterval(timeout, 1_000)
+const foo = new Foo()
+
+foo.doWork('success', (error, result) => {
+  assert.equal(error, undefined)
+  assert.equal(result, 'done')
+
+  assert.deepStrictEqual(context, {
+    start: true
+  })
+  delete context.start
+
+  clearInterval(timer1)
+})
+
+foo.doWork('fail', (error, result) => {
+  assert.equal(error.message, 'boom')
+  assert.equal(undefined, result)
+
+  assert.deepStrictEqual(context, {
+    end: true,
+    start: true
+  })
+  delete context.end
+  delete context.start
+
+  clearInterval(timer2)
+})
+
+function timeout() {
+  const current = process.hrtime.bigint()
+  if ((current - start) > 3e+10) {
+    // Exceeded 30 seconds, terminate it all.
+    clearInterval(timer1)
+    clearInterval(timer2)
+  }
+}
diff --git a/tests/common/mod.rs b/tests/common/mod.rs
index f3dc816..1a013e6 100644
--- a/tests/common/mod.rs
+++ b/tests/common/mod.rs
@@ -12,14 +12,42 @@ use swc::config::IsModule;
 static TEST_MODULE_NAME: &str = "undici";
 static TEST_MODULE_PATH: &str = "index.mjs";
 
+/// PackageImport represents metadata around a module that we are pretending
+/// was imported from a npm module. So, if we are pretending to test
+/// instrumentation of a module `@foo/bar`, the `module_name` would be
+/// `@foo/bar`, the `module_version` would be a simple version string like
+/// `0.1.0`, and the `file` would be some filename within that package like
+/// `index.js` or `lib/utils.js`.
+///
+/// This information will be used to match a unit test to the instrumentation.
+pub struct PackageImport {
+    pub module_name: String,
+    pub module_version: String,
+    pub file: String
+}
+
 pub fn transpile_and_test(test_file: &str, mjs: bool, config: Config) {
+    transpile_and_test_with_imports(test_file, mjs, config, &[]);
+}
+
+pub fn transpile_and_test_with_imports(test_file: &str, mjs: bool, config: Config, imports: &[PackageImport]) {
     let test_file = PathBuf::from(test_file);
     let test_dir = test_file.parent().expect("Couldn't find test directory");
 
     let file_path = PathBuf::from("index.mjs");
     let instrumentor = Instrumentor::new(config);
-    let mut instrumentations =
+    let mut instrumentations;
+    if imports.len() > 0 {
+        let import = &imports[0];
+         instrumentations = instrumentor.get_matching_instrumentations(
+            import.module_name.as_str(),
+            import.module_version.as_str(),
+            &PathBuf::from(&import.file)
+        );
+    } else {
+       instrumentations =
         instrumentor.get_matching_instrumentations(TEST_MODULE_NAME, "0.0.1", &file_path);
+    }
 
     let extension = if mjs { "mjs" } else { "js" };
     let instrumentable = test_dir.join(format!("mod.{extension}"));
diff --git a/tests/instrumentor_test.rs b/tests/instrumentor_test.rs
index 6e2f7e9..340aed4 100644
--- a/tests/instrumentor_test.rs
+++ b/tests/instrumentor_test.rs
@@ -5,6 +5,7 @@
 mod common;
 
 mod arguments_mutation;
+mod callback_cjs;
 mod class_expression_cjs;
 mod class_method_cjs;
 mod constructor_cjs;

From d68d2019fed860cc60c9791e0daab0b1cbcbd552 Mon Sep 17 00:00:00 2001
From: James Sumners <jsumners@newrelic.com>
Date: Wed, 20 Aug 2025 09:14:29 -0400
Subject: [PATCH 02/10] address nonsense lints

I will never agree to implicit returns or difficult to read
boolean checks.
---
 src/function_query.rs  |  3 ++-
 src/instrumentation.rs | 11 +++++++----
 2 files changed, 9 insertions(+), 5 deletions(-)

diff --git a/src/function_query.rs b/src/function_query.rs
index cef3599..1b75b0a 100644
--- a/src/function_query.rs
+++ b/src/function_query.rs
@@ -25,7 +25,8 @@ impl FunctionKind {
     pub fn is_async(&self) -> bool {
         matches!(self, FunctionKind::Async)
     }
-    
+
+    #[must_use]
     pub fn is_callback(&self) -> bool {
         matches!(self, FunctionKind::Callback)
     }
diff --git a/src/instrumentation.rs b/src/instrumentation.rs
index 3218779..3d1fe41 100644
--- a/src/instrumentation.rs
+++ b/src/instrumentation.rs
@@ -131,7 +131,7 @@ impl Instrumentation {
         let trace_statement = construct_trace_statement(
             &self.config,
             &id_name,
-            (&self.module_version).clone().unwrap_or(String::new())
+            self.module_version.clone().unwrap_or_default().as_str()
         );
 
         body.stmts = vec![
@@ -395,13 +395,16 @@ pub fn get_script_start_index(script: &Script) -> usize {
 /// ```js
 /// return tr_ch_amp$myChannel.traceSync(__apm$traced, { arguments, self: this, moduleVersion: "1.0.0" })
 /// ```
-fn construct_trace_statement(config: &InstrumentationConfig, channel_name: &String, mod_version: String) -> Stmt {
+#[allow(clippy::bool_comparison)]
+#[allow(clippy::needless_return)]
+fn construct_trace_statement(config: &InstrumentationConfig, channel_name: &str, mod_version: &str) -> Stmt {
     let mut ctx = "{ arguments, self: this }".to_string();
-    if mod_version.len() > 0 {
-        ctx = ["{ arguments, self: this, moduleVersion: \"", mod_version.as_str(), "\" }"].join("");
+    if mod_version.is_ascii() == false {
+        ctx = ["{ arguments, self: this, moduleVersion: \"", mod_version, "\" }"].join("");
     }
 
     let operator = ["tr_ch_apm$", channel_name, ".", config.function_query.kind().tracing_operator()].join("");
+    #[allow(clippy::needless_late_init)]
     let stmt_str;
     if config.function_query.kind().is_callback() == true {
         // TODO: read callback position from configuration

From 5870dbd7dae87e44e0234af1c8c2c37cd995a584 Mon Sep 17 00:00:00 2001
From: James Sumners <jsumners@newrelic.com>
Date: Wed, 20 Aug 2025 09:27:01 -0400
Subject: [PATCH 03/10] address formatting errors

---
 src/function_query.rs     | 4 ++--
 src/instrumentation.rs    | 7 ++++---
 tests/callback_cjs/mod.rs | 8 ++++----
 tests/common/mod.rs       | 3 ++-
 4 files changed, 12 insertions(+), 10 deletions(-)

diff --git a/src/function_query.rs b/src/function_query.rs
index 1b75b0a..9663cf2 100644
--- a/src/function_query.rs
+++ b/src/function_query.rs
@@ -17,7 +17,7 @@ pub(crate) enum FunctionType {
 pub enum FunctionKind {
     Sync,
     Async,
-    Callback
+    Callback,
 }
 
 impl FunctionKind {
@@ -36,7 +36,7 @@ impl FunctionKind {
         match self {
             FunctionKind::Sync => "traceSync",
             FunctionKind::Async => "tracePromise",
-            FunctionKind::Callback => "traceCallback"
+            FunctionKind::Callback => "traceCallback",
         }
     }
 }
diff --git a/src/instrumentation.rs b/src/instrumentation.rs
index 3d1fe41..8ada1cc 100644
--- a/src/instrumentation.rs
+++ b/src/instrumentation.rs
@@ -131,7 +131,7 @@ impl Instrumentation {
         let trace_statement = construct_trace_statement(
             &self.config,
             &id_name,
-            self.module_version.clone().unwrap_or_default().as_str()
+            self.module_version.clone().unwrap_or_default().as_str(),
         );
 
         body.stmts = vec![
@@ -141,7 +141,7 @@ impl Instrumentation {
                 "if (!$ch.hasSubscribers) return __apm$traced();" as Stmt,
                 ch = ch_ident
             ),
-            trace_statement
+            trace_statement,
         ];
 
         self.has_injected = true;
@@ -397,6 +397,7 @@ pub fn get_script_start_index(script: &Script) -> usize {
 /// ```
 #[allow(clippy::bool_comparison)]
 #[allow(clippy::needless_return)]
+#[rustfmt::skip]
 fn construct_trace_statement(config: &InstrumentationConfig, channel_name: &str, mod_version: &str) -> Stmt {
     let mut ctx = "{ arguments, self: this }".to_string();
     if mod_version.is_ascii() == false {
@@ -418,4 +419,4 @@ fn construct_trace_statement(config: &InstrumentationConfig, channel_name: &str,
         "$stmt" as Stmt,
         stmt = Ident::new(Atom::from(stmt_str), Span::default(), SyntaxContext::empty())
     )
-}
\ No newline at end of file
+}
diff --git a/tests/callback_cjs/mod.rs b/tests/callback_cjs/mod.rs
index 700d896..ca81f77 100644
--- a/tests/callback_cjs/mod.rs
+++ b/tests/callback_cjs/mod.rs
@@ -12,13 +12,13 @@ fn callback_cjs() {
             FunctionQuery::FunctionExpression {
                 expression_name: "doWork".to_string(),
                 kind: FunctionKind::Callback,
-                index: 0
-            }
+                index: 0,
+            },
         )),
         &[PackageImport {
             module_name: "foo".to_string(),
             module_version: "1.1.1".to_string(),
-            file: "index.js".to_string()
+            file: "index.js".to_string(),
         }],
     )
-}
\ No newline at end of file
+}
diff --git a/tests/common/mod.rs b/tests/common/mod.rs
index 1a013e6..8072b4d 100644
--- a/tests/common/mod.rs
+++ b/tests/common/mod.rs
@@ -23,13 +23,14 @@ static TEST_MODULE_PATH: &str = "index.mjs";
 pub struct PackageImport {
     pub module_name: String,
     pub module_version: String,
-    pub file: String
+    pub file: String,
 }
 
 pub fn transpile_and_test(test_file: &str, mjs: bool, config: Config) {
     transpile_and_test_with_imports(test_file, mjs, config, &[]);
 }
 
+#[rustfmt::skip]
 pub fn transpile_and_test_with_imports(test_file: &str, mjs: bool, config: Config, imports: &[PackageImport]) {
     let test_file = PathBuf::from(test_file);
     let test_dir = test_file.parent().expect("Couldn't find test directory");

From 19ffd6b2b95b45e54bb0045ef0e83eef5355dbe9 Mon Sep 17 00:00:00 2001
From: James Sumners <jsumners@newrelic.com>
Date: Wed, 20 Aug 2025 09:43:34 -0400
Subject: [PATCH 04/10] tweak snap

---
 src/instrumentation.rs                       | 2 +-
 tests/wasm/__snapshots__/tests.test.mjs.snap | 2 +-
 2 files changed, 2 insertions(+), 2 deletions(-)

diff --git a/src/instrumentation.rs b/src/instrumentation.rs
index 8ada1cc..d41f483 100644
--- a/src/instrumentation.rs
+++ b/src/instrumentation.rs
@@ -400,7 +400,7 @@ pub fn get_script_start_index(script: &Script) -> usize {
 #[rustfmt::skip]
 fn construct_trace_statement(config: &InstrumentationConfig, channel_name: &str, mod_version: &str) -> Stmt {
     let mut ctx = "{ arguments, self: this }".to_string();
-    if mod_version.is_ascii() == false {
+    if mod_version.is_empty() == false {
         ctx = ["{ arguments, self: this, moduleVersion: \"", mod_version, "\" }"].join("");
     }
 
diff --git a/tests/wasm/__snapshots__/tests.test.mjs.snap b/tests/wasm/__snapshots__/tests.test.mjs.snap
index 92eec37..ba6536e 100644
--- a/tests/wasm/__snapshots__/tests.test.mjs.snap
+++ b/tests/wasm/__snapshots__/tests.test.mjs.snap
@@ -38,7 +38,7 @@ module.exports = class Up {
             const __apm$wrapped = ()=>{
                 console.log('fetch');
             };
-            return __apm$wrapped.apply(null, __apm$original_args);
+            return __apm$wrapped.apply(this, __apm$original_args);
         };
         if (!tr_ch_apm$up_fetch.hasSubscribers) return __apm$traced();
         return tr_ch_apm$up_fetch.traceSync(__apm$traced, {

From 6f81ae8f14be21256d71fe7129d1828daf35f6e1 Mon Sep 17 00:00:00 2001
From: James Sumners <jsumners@newrelic.com>
Date: Wed, 20 Aug 2025 10:07:43 -0400
Subject: [PATCH 05/10] tweak snap

---
 tests/wasm/__snapshots__/tests.test.mjs.snap | 24 +++++---------------
 1 file changed, 6 insertions(+), 18 deletions(-)

diff --git a/tests/wasm/__snapshots__/tests.test.mjs.snap b/tests/wasm/__snapshots__/tests.test.mjs.snap
index ba6536e..11763e8 100644
--- a/tests/wasm/__snapshots__/tests.test.mjs.snap
+++ b/tests/wasm/__snapshots__/tests.test.mjs.snap
@@ -41,11 +41,7 @@ module.exports = class Up {
             return __apm$wrapped.apply(this, __apm$original_args);
         };
         if (!tr_ch_apm$up_fetch.hasSubscribers) return __apm$traced();
-        return tr_ch_apm$up_fetch.traceSync(__apm$traced, {
-            arguments,
-            self: this,
-            moduleVersion: "1.0.0"
-        });
+        return tr_ch_apm$up_fetch.traceSync(__apm$traced, { arguments, self: this, moduleVersion: "1.0.0" });
     }
 };
 ",
@@ -91,14 +87,10 @@ export class Up {
             const __apm$wrapped = ()=>{
                 console.log('fetch');
             };
-            return __apm$wrapped.apply(null, __apm$original_args);
+            return __apm$wrapped.apply(this, __apm$original_args);
         };
         if (!tr_ch_apm$up_fetch.hasSubscribers) return __apm$traced();
-        return tr_ch_apm$up_fetch.traceSync(__apm$traced, {
-            arguments,
-            self: this,
-            moduleVersion: "1.0.0"
-        });
+        return tr_ch_apm$up_fetch.traceSync(__apm$traced, { arguments, self: this, moduleVersion: "1.0.0" });
     }
 }
 ",
@@ -144,17 +136,13 @@ export class Up {
             const __apm$wrapped = (url)=>{
                 console.log('fetch');
             };
-            return __apm$wrapped.apply(null, __apm$original_args);
+            return __apm$wrapped.apply(this, __apm$original_args);
         };
         if (!tr_ch_apm$up_fetch.hasSubscribers) return __apm$traced();
-        return tr_ch_apm$up_fetch.traceSync(__apm$traced, {
-            arguments,
-            self: this,
-            moduleVersion: "1.0.0"
-        });
+        return tr_ch_apm$up_fetch.traceSync(__apm$traced, { arguments, self: this, moduleVersion: "1.0.0" });
     }
 }
 ",
-  "map": "{"version":3,"file":"module.js","sources":["module.ts"],"sourceRoot":"","names":[],"mappings":";;;AAEA,MAAM,CAAA,MAAO,EAAE;IACX,aAAA;;;;;;;;;YACI,OAAO,CAAC,GAAG,CAAC,aAAa,CAAC,CAAC;;;;;;;;;;;;;;;;IAC/B,CAAC;IACD,KAAK,IAAS,EAAA;;;mCAAR;gBACF,OAAO,CAAC,GAAG,CAAC,OAAO,CAAC,CAAC;;;;;;;;;;IACzB,CAAC;CACJ"}",
+  "map": "{"version":3,"file":"module.js","sources":["module.ts"],"sourceRoot":"","names":[],"mappings":";;;AAEA,MAAM,CAAA,MAAO,EAAE;IACX,aAAA;;;;;;;;;YACI,OAAO,CAAC,GAAG,CAAC,aAAa,CAAC,CAAC;;;;;;;;;;;;;;;;IAC/B,CAAC;IACD,KAAK,IAAS,EAAA;;;mCAAR;gBACF,OAAO,CAAC,GAAG,CAAC,OAAO,CAAC,CAAC;;;;;;IACzB,CAAC;CACJ"}",
 }
 `;

From 1c34a9a448181f5acb3459fdd5546ac07de8eafa Mon Sep 17 00:00:00 2001
From: James Sumners <jsumners@newrelic.com>
Date: Wed, 20 Aug 2025 13:29:35 -0400
Subject: [PATCH 06/10] add callback config

---
 src/config.rs             | 39 +++++++++++++++++++++++++++++++++++++--
 src/instrumentation.rs    |  4 ++--
 tests/callback_cjs/mod.rs |  3 ++-
 3 files changed, 41 insertions(+), 5 deletions(-)

diff --git a/src/config.rs b/src/config.rs
index 3726ab5..14f9f41 100644
--- a/src/config.rs
+++ b/src/config.rs
@@ -47,6 +47,23 @@ impl ModuleMatcher {
     }
 }
 
+#[cfg_attr(feature = "wasm", derive(tsify::Tsify))]
+#[cfg_attr(
+    feature = "serde",
+    derive(serde::Serialize, serde::Deserialize),
+    serde(rename_all = "camelCase")
+)]
+#[derive(Debug, Clone)]
+/// `CallbackConfig` represents details needed when wrapping a function that
+/// accepts a callback. These details will be used to construct the
+/// `tracingChannel.traceCallback` invocation.
+pub struct CallbackConfig {
+    /// `position` is the ordinal of the callback function within the wrapped
+    /// function's parameter list. A value of `-1` indicates that the callback
+    /// is the last parameter in the list. The value is zero based.
+    pub position: i32,
+}
+
 #[cfg_attr(
     feature = "serde",
     derive(serde::Serialize, serde::Deserialize),
@@ -62,16 +79,34 @@ pub struct InstrumentationConfig {
     pub channel_name: String,
     pub module: ModuleMatcher,
     pub function_query: FunctionQuery,
+    pub callback_config: CallbackConfig,
 }
 
+#[allow(clippy::needless_return)]
 impl InstrumentationConfig {
     #[must_use]
     pub fn new(channel_name: &str, module: ModuleMatcher, function_query: FunctionQuery) -> Self {
-        Self {
+        return Self::new_with_callback_config(
+            channel_name,
+            module,
+            function_query,
+            CallbackConfig { position: -1 },
+        );
+    }
+
+    #[must_use]
+    pub fn new_with_callback_config(
+        channel_name: &str,
+        module: ModuleMatcher,
+        function_query: FunctionQuery,
+        callback_config: CallbackConfig,
+    ) -> Self {
+        return Self {
             channel_name: channel_name.to_string(),
             module,
             function_query,
-        }
+            callback_config,
+        };
     }
 
     #[must_use]
diff --git a/src/instrumentation.rs b/src/instrumentation.rs
index d41f483..9bc72c2 100644
--- a/src/instrumentation.rs
+++ b/src/instrumentation.rs
@@ -408,9 +408,9 @@ fn construct_trace_statement(config: &InstrumentationConfig, channel_name: &str,
     #[allow(clippy::needless_late_init)]
     let stmt_str;
     if config.function_query.kind().is_callback() == true {
-        // TODO: read callback position from configuration
         // TODO: figure out how we can pass a `this` argument
-        stmt_str = ["return ", &*operator, "(__apm$traced, -1, ", ctx.as_str(), ", this, ...arguments)"].join("");
+        let pos = config.callback_config.position.to_string();
+        stmt_str = ["return ", &*operator, "(__apm$traced, ", pos.as_str(), ", ", ctx.as_str(), ", this, ...arguments)"].join("");
     } else {
         stmt_str = ["return ", &*operator, "(__apm$traced, ", ctx.as_str(), ")"].join("");
     }
diff --git a/tests/callback_cjs/mod.rs b/tests/callback_cjs/mod.rs
index ca81f77..acc9513 100644
--- a/tests/callback_cjs/mod.rs
+++ b/tests/callback_cjs/mod.rs
@@ -6,7 +6,7 @@ fn callback_cjs() {
     transpile_and_test_with_imports(
         file!(),
         false,
-        Config::new_single(InstrumentationConfig::new(
+        Config::new_single(InstrumentationConfig::new_with_callback_config(
             "foo_ch",
             ModuleMatcher::new("foo", ">=1.0.0", "index.js").unwrap(),
             FunctionQuery::FunctionExpression {
@@ -14,6 +14,7 @@ fn callback_cjs() {
                 kind: FunctionKind::Callback,
                 index: 0,
             },
+            CallbackConfig { position: 1 },
         )),
         &[PackageImport {
             module_name: "foo".to_string(),

From ffbdfb984c52e574c5da448ed0491ec80affd37f Mon Sep 17 00:00:00 2001
From: James Sumners <jsumners@newrelic.com>
Date: Tue, 26 Aug 2025 13:23:02 -0400
Subject: [PATCH 07/10] fix wasm crash

---
 Cargo.lock             | 1 +
 Cargo.toml             | 1 +
 src/config.rs          | 8 +++++++-
 src/instrumentation.rs | 2 --
 src/lib.rs             | 2 ++
 5 files changed, 11 insertions(+), 3 deletions(-)

diff --git a/Cargo.lock b/Cargo.lock
index 08e522d..a95489e 100644
--- a/Cargo.lock
+++ b/Cargo.lock
@@ -1151,6 +1151,7 @@ version = "0.1.0"
 dependencies = [
  "assert_cmd",
  "getrandom",
+ "log",
  "nodejs-semver",
  "serde",
  "swc",
diff --git a/Cargo.toml b/Cargo.toml
index 031f7cb..9cd9462 100644
--- a/Cargo.toml
+++ b/Cargo.toml
@@ -27,6 +27,7 @@ wasm-bindgen = { version = "0.2", optional = true }
 tsify = { version='0.5', features = ["js"], optional = true}
 # we need this to enable the js feature
 getrandom = { version = "*", features = ["wasm_js"], optional = true }
+log = "0.4.27"
 
 [dev-dependencies]
 assert_cmd = "2"
diff --git a/src/config.rs b/src/config.rs
index 14f9f41..e858bf9 100644
--- a/src/config.rs
+++ b/src/config.rs
@@ -79,10 +79,11 @@ pub struct InstrumentationConfig {
     pub channel_name: String,
     pub module: ModuleMatcher,
     pub function_query: FunctionQuery,
+    #[tsify(optional)]
+    #[serde(default = "InstrumentationConfig::empty_callback_config")]
     pub callback_config: CallbackConfig,
 }
 
-#[allow(clippy::needless_return)]
 impl InstrumentationConfig {
     #[must_use]
     pub fn new(channel_name: &str, module: ModuleMatcher, function_query: FunctionQuery) -> Self {
@@ -149,4 +150,9 @@ impl InstrumentationConfig {
     pub fn matches(&self, module_name: &str, version: &str, file_path: &PathBuf) -> bool {
         self.module.matches(module_name, version, file_path)
     }
+
+    #[must_use]
+    pub fn empty_callback_config() -> CallbackConfig {
+        return CallbackConfig{ position: -1 }
+    }
 }
diff --git a/src/instrumentation.rs b/src/instrumentation.rs
index 9bc72c2..6375c5b 100644
--- a/src/instrumentation.rs
+++ b/src/instrumentation.rs
@@ -395,8 +395,6 @@ pub fn get_script_start_index(script: &Script) -> usize {
 /// ```js
 /// return tr_ch_amp$myChannel.traceSync(__apm$traced, { arguments, self: this, moduleVersion: "1.0.0" })
 /// ```
-#[allow(clippy::bool_comparison)]
-#[allow(clippy::needless_return)]
 #[rustfmt::skip]
 fn construct_trace_statement(config: &InstrumentationConfig, channel_name: &str, mod_version: &str) -> Stmt {
     let mut ctx = "{ arguments, self: this }".to_string();
diff --git a/src/lib.rs b/src/lib.rs
index 0618089..5d2d704 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -13,6 +13,8 @@
 #![deny(clippy::complexity)]
 #![deny(clippy::correctness)]
 #![deny(clippy::unwrap_used)]
+#![allow(clippy::bool_comparison)]
+#![allow(clippy::needless_return)]
 
 /**
  * Unless explicitly stated otherwise all files in this repository are licensed under the Apache-2.0 License.

From f05c0f31e7fed57d8c3d2fdf515f8b35720ae00a Mon Sep 17 00:00:00 2001
From: James Sumners <jsumners@newrelic.com>
Date: Tue, 26 Aug 2025 13:27:55 -0400
Subject: [PATCH 08/10] just effin format on save!

---
 src/config.rs | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/src/config.rs b/src/config.rs
index e858bf9..51f3750 100644
--- a/src/config.rs
+++ b/src/config.rs
@@ -153,6 +153,6 @@ impl InstrumentationConfig {
 
     #[must_use]
     pub fn empty_callback_config() -> CallbackConfig {
-        return CallbackConfig{ position: -1 }
+        return CallbackConfig { position: -1 }
     }
 }

From d3fecdb72a6f482c798ddd02c4ed9300bab90892 Mon Sep 17 00:00:00 2001
From: James Sumners <jsumners@newrelic.com>
Date: Tue, 26 Aug 2025 13:33:03 -0400
Subject: [PATCH 09/10] =?UTF-8?q?=F0=9F=98=92?=
MIME-Version: 1.0
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 8bit

---
 src/config.rs | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/src/config.rs b/src/config.rs
index 51f3750..f579741 100644
--- a/src/config.rs
+++ b/src/config.rs
@@ -153,6 +153,6 @@ impl InstrumentationConfig {
 
     #[must_use]
     pub fn empty_callback_config() -> CallbackConfig {
-        return CallbackConfig { position: -1 }
+        return CallbackConfig { position: -1 };
     }
 }

From 1520740df90d92ae1d31dc960d31d849ab3e9df5 Mon Sep 17 00:00:00 2001
From: James Sumners <jsumners@newrelic.com>
Date: Wed, 27 Aug 2025 07:32:32 -0400
Subject: [PATCH 10/10] revert cargo files

---
 Cargo.lock | 1 -
 Cargo.toml | 1 -
 2 files changed, 2 deletions(-)

diff --git a/Cargo.lock b/Cargo.lock
index a95489e..08e522d 100644
--- a/Cargo.lock
+++ b/Cargo.lock
@@ -1151,7 +1151,6 @@ version = "0.1.0"
 dependencies = [
  "assert_cmd",
  "getrandom",
- "log",
  "nodejs-semver",
  "serde",
  "swc",
diff --git a/Cargo.toml b/Cargo.toml
index 9cd9462..031f7cb 100644
--- a/Cargo.toml
+++ b/Cargo.toml
@@ -27,7 +27,6 @@ wasm-bindgen = { version = "0.2", optional = true }
 tsify = { version='0.5', features = ["js"], optional = true}
 # we need this to enable the js feature
 getrandom = { version = "*", features = ["wasm_js"], optional = true }
-log = "0.4.27"
 
 [dev-dependencies]
 assert_cmd = "2"