# Explain a Stack Trace Line by Line

Source: https://coremesh.online/prompts/explain-a-stack-trace/

Explain this Node.js 20 stack trace to someone who is comfortable with JavaScript, new to this codebase:

```
TypeError: Cannot read properties of undefined (reading 'map')
    at renderList (src/list.js:14:22)
    at App (src/app.js:31:5)
```

1. Which frame is the real source of the problem, and why that one?
2. In one line each, what happened in the frames above it?
3. What are the two most likely causes?
4. Which file and line should I open first, and what should I check there?