# Visualization Node Click Fix - 2025-12-09

## Problem

Node clicks in the visualization were failing with a JavaScript error:

```
Uncaught TypeError: Cannot read properties of undefined (reading 'nodes')
    at updateStats ((index):3722:36)
    at renderGraphV2 ((index):2241:13)
    at showContentPane ((index):3884:17)
    at SVGGElement.handleNodeClickV2 ((index):1670:13)
```

**Symptoms:**
1. Click event was being detected: `[Click] Node clicked: directory src`
2. Render started: `[Render] Rendering graph, mode: tree_root phase: Phase 2 (radial)`
3. Crashed when `updateStats()` tried to read `.nodes` from undefined parameter

## Root Cause

The `updateStats()` function at line 1047 in `scripts.py` expected a `data` parameter with `nodes` and `links` properties:

```javascript
function updateStats(data) {
    const stats = d3.select("#stats");
    stats.html(`
        <div>Nodes: ${data.nodes.length}</div>  // ❌ Crashes here when data is undefined
        <div>Links: ${data.links.length}</div>
        ...
    `);
}
```

But at line 3688 in the `renderGraphV2()` function, it was being called **without any arguments**:

```javascript
// 5. Post-render updates
updateBreadcrumbsV2();
updateStats();  // ❌ No arguments passed
```

## Solution

**Two-part fix:**

### 1. Defensive Guard in `updateStats()` (lines 1048-1052)

Added null/undefined check to prevent crashes:

```javascript
function updateStats(data) {
    // Guard against undefined or null data
    if (!data || !data.nodes || !data.links) {
        console.warn('[Stats] updateStats called without valid data:', data);
        return;
    }

    const stats = d3.select("#stats");
    stats.html(`
        <div>Nodes: ${data.nodes.length}</div>
        <div>Links: ${data.links.length}</div>
        ${data.metadata ? `<div>Files: ${data.metadata.total_files || 'N/A'}</div>` : ''}
        ${data.metadata && data.metadata.is_monorepo ? `<div>Monorepo: ${data.metadata.subprojects.length} subprojects</div>` : ''}
    `);
    ...
}
```

### 2. Pass Correct Data in `renderGraphV2()` (lines 3694-3698)

Updated the call site to pass the required data object:

```javascript
// 5. Post-render updates
updateBreadcrumbsV2();
updateStats({
    nodes: visibleNodesList,
    links: visibleLinks,
    metadata: {total_files: allNodes.length}
});
```

This matches the pattern used elsewhere in the code (e.g., line 650):

```javascript
updateStats({nodes: visibleNodesList, links: visibleLinks, metadata: {total_files: allNodes.length}});
```

## Changes Made

**File:** `/Users/masa/Projects/mcp-vector-search/src/mcp_vector_search/cli/commands/visualize/templates/scripts.py`

1. **Lines 1048-1052**: Added defensive guard to check for valid data parameter
2. **Lines 3694-3698**: Updated `updateStats()` call to pass required data object

## Testing

### Automated Verification

All critical checks passed:

- ✅ Guard clause exists in `updateStats` function
- ✅ `updateStats` called with data object (2 occurrences)
- ✅ No calls to `updateStats()` without arguments
- ✅ Metadata parameter included in all calls
- ✅ Python syntax valid (`py_compile` passed)
- ✅ JavaScript template generation successful (159,947 chars)
- ✅ Visualization server app creation successful

### Test Results

```
============================================================
VERIFICATION: Node Click Fix
============================================================

✅ PASS: Guard clause exists in updateStats function
✅ PASS: updateStats called with data object (2 occurrence(s))
✅ PASS: No calls to updateStats() without arguments
✅ PASS: Metadata parameter included (2 occurrence(s))

============================================================
SUMMARY: All critical checks passed ✅
============================================================
```

## Expected Behavior

After this fix:

1. Node clicks should complete successfully
2. Graph should re-render without errors
3. Stats panel should update with correct node/link counts
4. If `updateStats()` is ever called without data, it logs a warning instead of crashing

## Design Pattern

This fix follows the defensive programming pattern:

- **Fail gracefully**: Check inputs before using them
- **Log warnings**: Help debugging by logging unexpected conditions
- **Consistent API usage**: Match existing patterns in the codebase

## Related Code

Other functions that call `updateStats()` correctly:
- Line 650: Force layout render function (passes full data object)

## Net Impact

- **Lines Changed**: 7 lines added (5 in guard, 2 in call site restructure)
- **LOC Delta**: +5 net (defensive code)
- **Bugs Fixed**: 1 (critical - blocking all node interactions)
