# Debugging Guide

This guide covers debugging with DAP (Debug Adapter Protocol) for TypeScript, Python, and Go.

## Overview

DAP provides IDE-like debugging:
- Breakpoints (line, conditional, logpoints)
- Step through code (into, over, out)
- Variable inspection
- Call stack navigation
- Expression evaluation

## Quick Start

### Basic Debugging Flow

1. **Set breakpoint**: `<leader>db`
2. **Start debugger**: `<leader>dc`
3. **Step through**: `<leader>di` (into), `<leader>do` (over)
4. **Inspect**: `<leader>du` (toggle UI)
5. **Stop**: `<leader>dt`

## Key Bindings

### Breakpoints

| Key | Action |
|-----|--------|
| `<leader>db` | Toggle breakpoint |
| `<leader>dB` | Conditional breakpoint |
| `<leader>dl` | Log point (print without stopping) |

### Execution Control

| Key | Action |
|-----|--------|
| `<leader>dc` | Continue (or start) |
| `<leader>di` | Step into |
| `<leader>do` | Step over |
| `<leader>dO` | Step out |
| `<leader>dC` | Run to cursor |
| `<leader>dp` | Pause |
| `<leader>dt` | Terminate |
| `<leader>dr` | Toggle REPL |

### UI & Inspection

| Key | Action |
|-----|--------|
| `<leader>du` | Toggle DAP UI |
| `<leader>de` | Evaluate expression |
| `<leader>dw` | Add to watch |
| `<leader>ds` | View scopes |

## DAP UI Layout

When you press `<leader>du`, the UI shows:

```
┌─────────────┬─────────────────────┐
│ Scopes      │                     │
│ (variables) │    Source Code      │
├─────────────┤    (your file)      │
│ Breakpoints │                     │
├─────────────┤                     │
│ Call Stack  │                     │
├─────────────┼─────────────────────┤
│ Watch       │    Console/REPL     │
└─────────────┴─────────────────────┘
```

### Scopes Panel

Shows variables in current scope:
- Local variables
- Function arguments
- Global variables
- Expand objects/arrays with `<cr>`

### Call Stack Panel

Shows function call hierarchy:
- Current function at top
- Click to jump to that frame
- See where you came from

### Breakpoints Panel

Lists all breakpoints:
- Toggle enable/disable
- Delete breakpoints
- See conditions

### Watch Panel

Add expressions to monitor:
- `<leader>dw` to add
- Updates as you step

## Debugging by Language

### TypeScript/JavaScript

#### Prerequisites

```bash
npm install -g @vscode/js-debug
```

#### Debug Current File

1. Open your `.ts` or `.js` file
2. `<leader>db` - Set breakpoint
3. `<leader>dc` - Start debugging
4. Select "Node.js: Launch" or similar

#### Debug Tests

```
<leader>td    Debug test at cursor
```

This automatically sets up the test runner with debugging.

#### launch.json Example

```json
{
  "version": "0.2.0",
  "configurations": [
    {
      "type": "pwa-node",
      "request": "launch",
      "name": "Debug Current File",
      "program": "${file}",
      "cwd": "${workspaceFolder}",
      "sourceMaps": true
    },
    {
      "type": "pwa-node",
      "request": "launch",
      "name": "Debug Server",
      "program": "${workspaceFolder}/src/index.ts",
      "preLaunchTask": "tsc: build",
      "outFiles": ["${workspaceFolder}/dist/**/*.js"]
    }
  ]
}
```

### Python

#### Prerequisites

```bash
pip install debugpy
```

#### Debug Current File

1. Open your `.py` file
2. `<leader>db` - Set breakpoint
3. `<leader>dc` - Start debugging
4. Select "Python: Current File"

#### Debug Tests

```
<leader>td    Debug test at cursor
```

#### launch.json Example

```json
{
  "version": "0.2.0",
  "configurations": [
    {
      "name": "Python: Current File",
      "type": "python",
      "request": "launch",
      "program": "${file}",
      "console": "integratedTerminal"
    },
    {
      "name": "Python: Flask",
      "type": "python",
      "request": "launch",
      "module": "flask",
      "args": ["run", "--debug"],
      "env": {
        "FLASK_APP": "app.py"
      }
    },
    {
      "name": "Python: Django",
      "type": "python",
      "request": "launch",
      "program": "${workspaceFolder}/manage.py",
      "args": ["runserver", "--noreload"]
    }
  ]
}
```

### Go

#### Prerequisites

```bash
go install github.com/go-delve/delve/cmd/dlv@latest
```

#### Debug Current File

1. Open your `.go` file
2. `<leader>db` - Set breakpoint
3. `<leader>dc` - Start debugging
4. Select "Go: Debug Package"

#### Debug Tests

```
<leader>td    Debug test at cursor
```

Works with table-driven tests too!

#### launch.json Example

```json
{
  "version": "0.2.0",
  "configurations": [
    {
      "name": "Debug Package",
      "type": "go",
      "request": "launch",
      "mode": "debug",
      "program": "${fileDirname}"
    },
    {
      "name": "Debug Main",
      "type": "go",
      "request": "launch",
      "mode": "debug",
      "program": "${workspaceFolder}/cmd/server"
    },
    {
      "name": "Attach to Process",
      "type": "go",
      "request": "attach",
      "mode": "local",
      "processId": "${command:pickProcess}"
    }
  ]
}
```

## Breakpoint Types

### Line Breakpoint

Standard breakpoint - stops when line is reached:
```
<leader>db    Toggle on current line
```

### Conditional Breakpoint

Only stops when condition is true:
```
<leader>dB    Prompts for condition
```

Example conditions:
- `x > 10`
- `user.name == "test"`
- `items.length > 0`

### Log Point

Prints message without stopping:
```
<leader>dl    Prompts for log message
```

Use `{}` for interpolation:
- `Value of x: {x}`
- `User: {user.name}`

### Hit Count Breakpoint

Stop after N hits (set via conditional):
```
<leader>dB    Enter: hitCount > 5
```

## Debugging Strategies

### Step Through Unknown Code

1. Set breakpoint at entry point
2. `<leader>dc` - Start
3. `<leader>di` - Step into functions to understand them
4. `<leader>do` - Step over when you understand
5. `<leader>dO` - Step out when done with function

### Find Where Value Changes

1. Set breakpoint where value is used
2. `<leader>dw` - Add variable to watch
3. Step through, watching for changes
4. Or use conditional breakpoint: `x != expectedValue`

### Debug Loops

1. Set conditional breakpoint: `i == problematicIndex`
2. Or use hit count: stop after N iterations
3. Check variables at specific iteration

### Remote Debugging

For processes running elsewhere:

1. Start process with debug flag
2. Use "attach" configuration
3. Connect to debug port

## Common Workflows

### Finding a Bug

1. Reproduce the bug path
2. Set breakpoint before bug occurs
3. `<leader>dc` - Start debugging
4. Step through, inspecting variables
5. Find where actual != expected
6. Fix and re-test

### Understanding New Code

1. Set breakpoint at function entry
2. `<leader>dc` - Start with a test case
3. `<leader>di` - Step into each function
4. Use watch to track key variables
5. Build mental model of flow

### Performance Investigation

Use log points to trace without stopping:
1. `<leader>dl` at key points
2. Log timing: `Start: {Date.now()}`
3. Run normally
4. Check console for timing

## Tips

### Quick Variable Inspection

Hover over variable with cursor - shows value in float.

### REPL Usage

`<leader>dr` opens REPL where you can:
- Evaluate expressions
- Call functions
- Modify variables (sometimes)

### Restart Debugging

If you need to start over:
```
<leader>dt    Terminate
<leader>dc    Start again
```

### Breakpoint Persistence

Breakpoints are saved per session. They'll be there when you reopen the file.

## Troubleshooting

### Debugger Won't Start

1. Check adapter is installed
2. `:DapShowLog` for errors
3. Verify launch configuration

### Breakpoints Not Hit

1. Verify file paths match
2. Check source maps (TypeScript)
3. Ensure code is actually executed

### Variables Not Showing

1. Might be optimized out (Go)
2. Check scope (local vs global)
3. Try evaluating directly: `<leader>de`

### Source Maps Issues (TypeScript)

1. Ensure `sourceMaps: true` in launch.json
2. Check tsconfig has `sourceMap: true`
3. Verify outFiles pattern matches

### Slow Debugging

1. Reduce number of breakpoints
2. Use conditional breakpoints
3. Disable logging breakpoints when not needed