A chess game against an AI opponent, built with React, TypeScript and Vite.
Play it here: https://bell-kevin.github.io/chess/
The engine implements the full rules of chess:
- Castling, both sides, with every restriction: the king and rook must be unmoved, the squares between them empty, and the king may not castle out of, through, or into check.
- En passant, including the case where the capture is illegal because it would expose the king along the rank.
- Promotion to a queen, rook, bishop or knight — under-promotion is offered rather than assumed.
- Draws: stalemate, insufficient material, the fifty-move rule and threefold repetition.
Move generation is verified against the standard perft node counts to depth 5 from six reference positions, which is what makes the castling and en passant corner cases trustworthy rather than merely untested.
The bot searches with negamax and alpha-beta pruning, driven by iterative deepening under a wall-clock budget so it always answers promptly:
- Quiescence search extends along captures and promotions, so the bot never evaluates a position in the middle of a trade and hands you a piece.
- Move ordering by most-valuable-victim / least-valuable-attacker, plus killer moves, which is what makes the pruning effective.
- Evaluation combines material, piece-square tables (with a separate king table for the endgame), the bishop pair, and doubled, isolated and passed pawns.
- Mate scores fold in the distance to mate, so the bot prefers the quickest win and the most stubborn defence.
The search runs in a Web Worker, so even the deepest setting never freezes the board — which matters most on a phone.
| Level | Behaviour |
|---|---|
| Very Easy | Random legal moves — for learning how the pieces move |
| Easy | One-ply search, picks loosely among reasonable moves and blunders often |
| Casual | Two-ply search without quiescence — sees the reply to its own move, but not the trade that follows |
| Medium | Two-ply search with quiescence and a little randomness |
| Hard | Four-ply search; punishes hanging pieces |
| Very Hard | Six-ply iterative deepening; will not give away material |
Win mode turns the engine on your own position and tells you what to play. It is off by default; the switch sits at the top of the side panel.
While it is on, every one of your turns is searched at full strength, whatever difficulty the game is set to — the point of a hint is that it is the best move on the board, not the move a beginner-level opponent would find. The recommendation is shown three ways at once, so it lands whether or not you read notation:
- an arrow on the board, from the piece to the square it should go to,
- the move in algebraic notation, and the same move in words ("move your knight from g1 to f3, taking the bishop"),
- what it is worth — a forced mate is counted in moves, anything else is scored in pawns, along with the line the engine expects to follow.
It says so plainly when a position is not winning: a mode that called every move a winning one would only be lying about the game you are in.
The hint search runs on a worker of its own, separate from the one the opponent thinks on, so asking for hints never delays the bot's own replies. A hint you have already moved past is abandoned outright rather than left running on a board nobody is looking at.
The board is keyboard accessible: it is a single tab stop, the arrow keys move between squares, and Enter selects or plays. Every square carries a label for screen readers. The layout adapts to portrait phones, landscape phones, tablets and desktop.
npm install
npm run dev # start the dev server
npm test # rules, perft and bot tests
npm run typecheck
npm run lint
npm run buildEvery push to main builds the app and publishes it to GitHub Pages via
.github/workflows/deploy.yml, which gates the
deploy on typecheck and the test suite. The site then updates on its own; no
gh-pages branch is involved. A fork needs Settings → Pages → Build and
deployment set to GitHub Actions once before its first deploy.
The Vite build uses base: './', so the same dist/ works whether it is served
from the project path (/chess/), a domain root, or a custom domain. To serve
it from a custom domain, point the DNS at GitHub Pages and set the domain under
Settings → Pages.
Play it here: https://bell-kevin.github.io/chess/
or here: