js-virtual-dom-diff

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js-virtual-dom-diff

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Answer 1

The implementation is in ~/vdom.js. Here's the architecture:

Core Algorithm

The engine uses a recursive diff with keyed child reconciliation for O(n) performance:

diff(oldNode, newNode, path = '', patches = {}) → patches map { path: patch[] }

Patch types (PATCH_TYPES): - CREATE — insert a new node where none existed - REPLACE — swap an existing node for a completely new one (different tag or type) - UPDATE_ATTRS — modify attributes on an existing node (same tag) - REMOVE — delete a node - REORDER — structural changes to children (CREATE/REMOVE/MOVE/REPLACE sub-operations)

Keyed Reconciliation (O(n))

When children have key attributes, the algorithm avoids O(n²) naive differencing:

reconcileChildren(oldChildren, newChildren) → operations[]

Build key→index maps for both old and new child lists
Remove keys absent in the new list → REMOVE ops
Create keys absent in the old list → CREATE ops
For keys present in both, find the Longest Increasing Subsequence (LIS) of new-indices (in old order) → these stay in place
Remaining kept nodes → MOVE ops

LIS algorithm (O(n log n)) uses patience sorting with traceback reconstruction to minimize DOM moves.

Non-keyed Fallback

When no keys are present, the engine falls back to index-based comparison — matching children at the same position and recursing when tags match.

Patching

patch(rootNode, patches) applies the patches map to a real DOM (or mock DOM in Node): - Processes patches by path top-down - For REORDER operations: applies removals (descending), then creates, then moves via insertBefore - Recurses into children for sub-path patches

Key Code Snippets

// Core diff — decide what to patch
function diff(oldNode, newNode, path = '', patches = {}) {
  if (oldNode === newNode) return patches;
  if (oldNode == null && newNode != null)  → CREATE
  if (oldNode != null && newNode == null)  → REMOVE
  if (typeof oldNode !== typeof newNode || oldNode.tag !== newNode.tag) → REPLACE
  // Same tag → diff attrs, reconcile children
  diffAttrs(...) → UPDATE_ATTRS
  reconcileChildren(...) → REORDER
  // Recurse into kept children (same key + same tag)
}

// Keyed reconciliation with LIS
function reconcileChildren(oldChildren, newChildren) {
  // Build key maps
  // REMOVE: keys in old but not in new
  // CREATE: keys in new but not in old  
  // LIS on new-indices of kept nodes → stay in place
  // MOVE: kept nodes not in LIS
}

Evidence & signatures

All **59 assertions** pass across 9 test suites:

| Suite | Tests | Coverage |
|---|---|---|
| Basic Diffing | 7 | identical trees, null→CREATE, node→REMOVE, tag→REPLACE, attr add/change/remove |
| Text Nodes | 2 | same/different string comparison |
| Children (no keys) | 3 | add, remove, replace child tags |
| Keyed Reconciliation | 6 | reorder, add middle, remove middle, swap, attr update, full list transform (5→5, one removed, one created, two moved) |
| Nested Diffing | 2 | deeply nested attr change, deeply nested tag replacement |
| Edge Cases | 8 | empty arrays, 0→many, many→0, null children, key filtering, same-reference optimization, insert first/last |
| LIS Algorithm | 5 | empty, single, increasing, decreasing, mixed |
| Patching (mock DOM) | 1 | validates full patch structure with attrs + REORDER + nested REPLACE |
| Performance | 2 | **1000-item keyed diff in 3ms**, 500-item non-keyed in 1ms |

### Edge Cases Tested

- **Same-reference optimization**: `diff(obj, obj)` returns empty patches (no work)
- **`key` attribute filtering**: keys are used for reconciliation but excluded from DOM attribute patches
- **Null children**: treated as empty arrays — no spurious patches
- **Text nodes as children**: string-to-string comparison produces REPLACE inside REORDER
- **Insert at beginning/end**: minimal CREATE operations (no unnecessary moves)
- **Full reverse with LIS**: reversing a 1000-item list produces only 999 MOVE ops (no CREATE/REMOVE)

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{"model": "Sonnet 3.7", "problem_class": "js-virtual-dom-diff", "result": "passed", "tests": 59}

Answer 2