Stack Trace

# Stack Trace Analysis Guide How to read and analyze stack traces across different languages. ## Anatomy of a Stack Trace A stack trace shows the call sequence that led to an error: ``` Error: Something went wrong at functionC (file3.js:10:5) <- Error thrown here at functionB (file2.js:20:3) <- Called by functionB at functionA (file1.js:30:7) <- Called by functionA at main (index.js:5:1) <- Entry point ``` **Reading Order**: Top = where error occurred, Bottom = entry point ## JavaScript/Node.js Stack Traces ### Standard Format ``` TypeError: Cannot read property 'name' of undefined at getUser (/app/src/services/user.js:45:23) at async handler (/app/src/routes/api.js:12:18) at Layer.handle [as handle_request] (/app/node_modules/express/lib/router/layer.js:95:5) at next (/app/node_modules/express/lib/router/route.js:144:13) ``` **Components**: - `TypeError` - Error type - `Cannot read property 'name' of undefined` - Error message - `/app/src/services/user.js:45:23` - File:Line:Column - `getUser` - Function name - `async handler` - Async function indicator ### Key Patterns **User Code vs Framework**: ``` at getUser (/app/src/services/user.js:45:23) <- YOUR CODE at handler (/app/src/routes/api.js:12:18) <- YOUR CODE at Layer.handle [as handle_request] (express/...) <- FRAMEWORK at next (express/lib/router/route.js:144:13) <- FRAMEWORK ``` Focus on frames in YOUR code first. **Anonymous Functions**: ``` at Object. (/app/index.js:5:1) at Array.forEach () at /app/src/utils.js:10:5 ``` Add function names for better traces: ```javascript // Instead of const handler = () => { ... } // Use const handler = function userHandler() { ... } ``` **Async Stack Traces** (Node.js 12+): ``` Error: Failed at fetchData (/app/src/api.js:10:9) at async main (/app/src/index.js:5:3) // -- async gap -- at Object. (/app/src/index.js:1:1) ``` Enable: `node --async-stack-traces app.js` --- ## Python Stack Traces ### Standard Format (Traceback) ```python Traceback (most recent call last): File "/app/main.py", line 10, in result = process_data(data) File "/app/processor.py", line 25, in process_data return transform(data['items']) File "/app/transformer.py", line 15, in transform return [parse(item) for item in items] File "/app/transformer.py", line 15, in return [parse(item) for item in items] KeyError: 'name' ``` **Reading Order**: Top = entry point, Bottom = error (opposite of JS!) **Components**: - `File "/app/processor.py"` - File path - `line 25` - Line number - `in process_data` - Function name - `return transform(data['items'])` - Actual code line ### Key Patterns **Chained Exceptions** (Python 3): ```python Traceback (most recent call last): File "app.py", line 5, in main data = json.loads(text) json.JSONDecodeError: Expecting value During handling of the above exception, another exception occurred: Traceback (most recent call last): File "app.py", line 8, in main raise ValueError("Invalid JSON input") from e ValueError: Invalid JSON input ``` Read both tracebacks - first is root cause, second is re-raised. **List Comprehension**: ```python File "/app/transformer.py", line 15, in ``` Error inside list comprehension - check the item being processed. --- ## Java Stack Traces ### Standard Format ```java java.lang.NullPointerException: Cannot invoke method on null at com.example.UserService.getUser(UserService.java:45) at com.example.ApiController.handleRequest(ApiController.java:23) at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method) at org.springframework.web.servlet.FrameworkServlet.service(FrameworkServlet.java:897) ... 20 more ``` **Components**: - `java.lang.NullPointerException` - Exception class - `com.example.UserService.getUser` - Package.Class.Method - `UserService.java:45` - File:Line - `... 20 more` - Truncated frames (same as parent) ### Key Patterns **Caused By Chain**: ```java Exception in thread "main" RuntimeException: Processing failed at App.main(App.java:10) Caused by: SQLException: Connection refused at DB.connect(DB.java:25) ... 5 more Caused by: SocketException: Connection reset at Socket.connect(Socket.java:100) ... 3 more ``` Root cause is at the bottom "Caused by". **Lambda Expressions**: ```java at App.lambda$main$0(App.java:15) ``` Lambda defined in `main` method. --- ## Go Stack Traces ### Standard Format ```go panic: runtime error: index out of range [5] with length 3 goroutine 1 [running]: main.processItems(0xc0000a4000, 0x3, 0x3) /app/main.go:25 +0x5e main.main() /app/main.go:10 +0x3a exit status 2 ``` **Components**: - `panic: runtime error` - Error type - `goroutine 1 [running]` - Which goroutine - `main.processItems` - Package.Function - `(0xc0000a4000, 0x3, 0x3)` - Arguments (pointers, values) - `/app/main.go:25 +0x5e` - File:Line + PC offset ### Key Patterns **Multiple Goroutines**: ```go goroutine 1 [running]: ... goroutine 5 [chan receive]: ... goroutine 6 [IO wait]: ... ``` Error in goroutine 1, others show their state. --- ## Rust Stack Traces ### Standard Format ```rust thread 'main' panicked at 'index out of bounds', src/main.rs:15:5 stack backtrace: 0: rust_begin_unwind 1: core::panicking::panic_fmt 2: core::panicking::panic_bounds_check 3: <usize as core::slice::index::SliceIndex>::index 4: my_app::process_data at ./src/processor.rs:25:10 5: my_app::main at ./src/main.rs:10:5 ``` Enable full backtraces: `RUST_BACKTRACE=1` --- ## Analysis Workflow ### Step 1: Find the Error - **JavaScript**: Top of stack - **Python**: Bottom of traceback - **Java/Go/Rust**: Start of panic/exception ### Step 2: Identify Your Code Skip framework/library frames, focus on your code: ``` at getUser (YOUR CODE) <- Focus here at express.Router (FRAMEWORK) <- Skip at mongoose.Query (LIBRARY) <- Skip ``` ### Step 3: Extract Key Info | What | Example | |------|---------| | Error Type | `TypeError`, `NullPointerException` | | Message | `Cannot read property 'x' of undefined` | | File | `/app/src/user.js` | | Line | `45` | | Function | `getUser` | | Value | `undefined` | ### Step 4: Read the Context ```javascript // Line 45 const userName = user.profile.name // ^--- user is undefined ``` ### Step 5: Trace the Source Go up the stack: 1. Where is `user` defined? 2. Where does it come from? 3. Why is it undefined? ```javascript // In API handler (line 12) const user = await getUserById(id) // Returns undefined if not found! return response.json({ name: user.profile.name }) ``` --- ## Common Stack Trace Issues ### Minified Code ``` TypeError: n is not a function at e (bundle.js:1:12345) at t (bundle.js:1:23456) ``` **Solutions**: 1. Use source maps 2. Run unminified build for debugging 3. Add `devtool: 'source-map'` to webpack ### Missing Stack Trace ``` Error: Something went wrong at Object. (index.js:1:1) ``` **Causes**: - Error created with `new Error()` but not thrown - Stack trimmed by error handling - Async context lost ### Infinite/Very Deep Stack ``` RangeError: Maximum call stack size exceeded at recursive (app.js:10:3) at recursive (app.js:12:5) at recursive (app.js:12:5) ... (repeating) ``` **Cause**: Infinite recursion - find base case issue. --- ## Tools ### Node.js ```bash # Enable long stack traces node --stack-trace-limit=100 app.js # V8 flags for debugging node --trace-warnings app.js ``` Note: Async stack traces are enabled by default in Node.js 12+. ### Python ```bash # Low-level crash debugging python -X faulthandler script.py ``` ```python # Rich traceback (pip install rich) from rich import traceback traceback.install() ``` ### Browser ```javascript // Get stack trace anywhere console.trace('Here') // Error with custom stack const err = new Error('Debug') console.log(err.stack) ```