Frontend Architecture

Overview

Lichess’s frontend is built with TypeScript and Snabbdom, a lightweight virtual DOM library. The UI is organized as a pnpm monorepo with 35+ packages, each handling specific features. The architecture emphasizes performance, modularity, and real-time updates.

Technology Stack

Core Technologies

TypeScript 5.9+: Type-safe JavaScript development
Snabbdom 3.5.1: Minimal virtual DOM library for efficient rendering
esbuild: Ultra-fast JavaScript bundler
pnpm: Fast, disk-efficient package manager
Chessground: Custom chess board UI component
Sass: CSS preprocessing

Key Libraries

// package.json
{
  "dependencies": {
    "@lichess-org/chessground": "^10.0.2",
    "@lichess-org/pgn-viewer": "^2.5.9",
    "snabbdom": "3.5.1",
    "chessops": "^0.15",
    "typescript": "^5.9.3"
  }
}

Monorepo Structure

The UI is organized as a pnpm workspace defined in /pnpm-workspace.yaml:

ui/
├── .build/          # Build system (esbuild configuration)
├── @types/          # TypeScript type definitions
├── analyse/         # Analysis board
├── round/           # Live game interface
├── lobby/           # Game lobby
├── tournament/      # Tournament UI
├── puzzle/          # Tactics trainer
├── study/           # Study/analysis sharing (via analyse)
├── site/            # Core site functionality
├── lib/             # Shared utilities and components
├── bits/            # Small reusable UI components
└── [30+ more packages]

Package Dependencies

Packages declare dependencies using workspace protocol:

// ui/analyse/package.json
{
  "name": "analyse",
  "dependencies": {
    "lib": "workspace:*",
    "@lichess-org/chessground": "^10.0.2",
    "snabbdom": "3.5.1"
  }
}

Virtual DOM with Snabbdom

Why Snabbdom?

Lichess chose Snabbdom for its:

Minimal size: ~200 lines of core code
High performance: Fast diffing algorithm
Flexibility: Extensible module system
Simplicity: Easy to understand and debug

Snabbdom Basics

Snabbdom uses a hyperscript function h() to create virtual DOM nodes:

import { h, type VNode } from 'snabbdom';

// Create virtual DOM node
const vnode: VNode = h('div.game-board', {
  attrs: { 'data-game-id': gameId },
  on: { click: handleClick }
}, [
  h('span.player', playerName),
  h('div.board', renderBoard())
]);

View Functions

UI components are pure functions returning VNodes:

// ui/analyse/src/view/main.ts
import { type VNode } from 'lib/view';
import type AnalyseCtrl from '../ctrl';

export default function (deps?: typeof studyDeps) {
  return function (ctrl: AnalyseCtrl): VNode {
    if (ctrl.nvui) return ctrl.nvui.render(deps);
    else if (deps && ctrl.study) return studyView(ctrl, ctrl.study, deps);
    else return analyseView(ctrl, deps);
  };
}

function analyseView(ctrl: AnalyseCtrl, deps?: typeof studyDeps): VNode {
  const ctx = viewContext(ctrl, deps);
  return renderMain(
    ctx,
    renderBoard(ctx),
    renderTools(ctx),
    renderControls(ctrl),
    renderUnderboard(ctx)
  );
}

Rendering Cycle

State change: User action or server event updates controller state
Redraw trigger: Controller calls redraw() function
View render: View function generates new VNode tree
Diff & patch: Snabbdom diffs against previous VNode and patches DOM

// Typical controller pattern
export class AnalyseCtrl {
  redraw: () => void;  // Injected by bootstrap
  
  makeMove(move: Move): void {
    this.tree.addMove(move);
    this.redraw();  // Triggers re-render
  }
}

UI Package Architecture

Core Pattern: MVC-style Controllers

Most UI packages follow an MVC-inspired pattern:

analyse/
├── src/
│   ├── ctrl.ts          # Main controller class
│   ├── interfaces.ts    # TypeScript interfaces
│   ├── view/
│   │   ├── main.ts      # Root view function
│   │   ├── controls.ts  # Sub-views
│   │   └── components.ts
│   ├── socket.ts        # WebSocket integration
│   ├── boot.ts          # Initialization
│   └── analyse.ts       # Entry point

Controller Example

From ui/analyse/src/ctrl.ts:

import { TreeWrapper } from 'lib/tree';
import { CevalCtrl } from 'lib/ceval';
import { Socket } from './socket';

export default class AnalyseCtrl {
  data: AnalyseData;
  tree: TreeWrapper;
  ceval: CevalCtrl;
  socket: Socket;
  ground: ChessgroundApi;  // Chess board
  
  redraw: () => void;
  
  constructor(opts: AnalyseOpts, redraw: () => void) {
    this.data = opts.data;
    this.redraw = redraw;
    this.tree = makeTree(opts.treeParts);
    this.ceval = new CevalCtrl(opts.ceval, this);
    this.socket = makeSocket(opts.socketSend, this);
  }
  
  jump(path: Tree.Path): void {
    this.tree.setPath(path);
    this.updateBoard();
    this.redraw();
  }
  
  private updateBoard(): void {
    const node = this.tree.getCurrentNode();
    this.ground.set({
      fen: node.fen,
      lastMove: node.uci
    });
  }
}

Bootstrap Pattern

Each page bootstraps its UI from server-provided data:

// ui/analyse/src/boot.ts
import { init as snabbdomInit } from 'snabbdom';
import makeCtrl from './ctrl';
import view from './view';

export function boot(opts: AnalyseOpts) {
  const patch = snabbdomInit([...snabbdomModules]);
  
  let vnode: VNode;
  const redraw = () => {
    vnode = patch(vnode, view(ctrl));
  };
  
  const ctrl = makeCtrl(opts, redraw);
  const element = document.querySelector('.analyse')!;
  vnode = patch(element, view(ctrl));
}

// Server embeds bootstrap code:
// <script>LichessAnalyse.boot({...data})</script>

Shared Library (`ui/lib`)

The lib package provides common utilities used across all UI packages:

Core Utilities

// ui/lib/src/index.ts exports:

// Functional helpers
export const prop = <T>(value: T): Prop<T> => {...};
export const toggle = (initial: boolean): Toggle => {...};
export const defined = <T>(v: T | undefined): v is T => v !== undefined;

// Async utilities
export const requestIdleCallback = (f: () => void) => {...};
export const debounce = <F extends (...args: any[]) => void>(
  func: F, wait: number
) => {...};

// View helpers
export { h, type VNode } from 'snabbdom';
export const onInsert = (f: (el: HTMLElement) => void) => {...};

Game Utilities

// ui/lib/src/game/

export const playable = (game: Game): boolean => 
  game.status.id < 25 && !game.player.spectator;

export const validUci = (fen: string, uci: Uci): boolean => {...};

export const fenToEpd = (fen: string): string => {...};

Chess Evaluation (ceval)

Client-side Stockfish integration:

// ui/lib/src/ceval/
export class CevalCtrl {
  start(path: Tree.Path, nodes: Tree.Node[]): void {
    this.worker.postMessage({
      type: 'start',
      fen: nodes[nodes.length - 1].fen,
      moves: this.getMoves(nodes)
    });
  }
}

Chessground Integration

Chessground is Lichess’s custom-built chess board component:

import { Chessground } from '@lichess-org/chessground';
import type { Api as ChessgroundApi } from '@lichess-org/chessground/api';

export function makeBoard(element: HTMLElement): ChessgroundApi {
  return Chessground(element, {
    fen: 'rnbqkbnr/pppppppp/8/8/8/8/PPPPPPPP/RNBQKBNR',
    orientation: 'white',
    movable: {
      free: false,
      color: 'white',
      dests: new Map()  // Legal moves
    },
    events: {
      move: (orig, dest) => handleMove(orig, dest)
    }
  });
}

Chessground features:

SVG-based piece rendering
Touch and mouse input
Premoves and conditional moves
Animation and highlighting
Mobile-optimized

Build System

The custom build system in ui/.build/ uses esbuild:

Package Configuration

Each package defines build configuration in package.json:

{
  "name": "analyse",
  "build": {
    "bundle": "src/**/analyse.*ts",
    "sync": {
      "node_modules/*stockfish*/*.{js,wasm}": "/public/npm"
    },
    "hash": ["/public/images/board/*.svg"]
  }
}

Build Properties

bundle: Entry points for esbuild

Matches analyse.ts, analyse.study.ts, etc.
Output: /public/compiled/analyse.[hash].js

sync: Copy assets to public directory

Useful for large files (Stockfish WASM)
Watch mode syncs on changes

hash: Content-hash assets for CDN caching

Creates symlinks in /public/hashed/
Manifest tracks hashes for server

Build Commands

ui/build              # Build all packages
ui/build -w           # Watch mode
ui/build analyse      # Build specific package
ui/build --help       # Show options

Code Splitting

esbuild automatically splits shared code into chunks:

public/compiled/
├── analyse.abc123.js        # Analyse entry point
├── round.def456.js          # Round entry point
└── lib.789xyz.js            # Shared lib chunk

Browsers cache shared chunks across pages.

TypeScript Configuration

Base TypeScript config in ui/tsconfig.base.json:

{
  "compilerOptions": {
    "target": "ES2022",
    "module": "ES2022",
    "moduleResolution": "bundler",
    "strict": true,
    "noUncheckedIndexedAccess": true,
    "paths": {
      "lib/*": ["./lib/src/*"],
      "bits/*": ["./bits/src/*"]
    }
  }
}

Each package extends base config with package-specific settings.

State Management

Lichess doesn’t use Redux or similar state libraries. State is managed in controllers:

Local State Pattern

export class PuzzleCtrl {
  // Direct properties
  mode: 'play' | 'view' = 'play';
  streak: number = 0;
  
  // Reactive properties (Prop pattern)
  loading = prop(false);
  showSolution = toggle(false);
  
  // Methods modify state and trigger redraw
  solve(): void {
    this.mode = 'view';
    this.streak++;
    this.showSolution(true);
    this.redraw();
  }
}

Communication Between Components

Components communicate via:

Direct calls: Parent controller calls child methods
Callbacks: Child triggers parent callback
PubSub: lib/pubsub for global events

import { pubsub } from 'lib/pubsub';

// Publish event
pubsub.emit('game.move', move);

// Subscribe to event
pubsub.on('game.move', (move) => handleMove(move));

Performance Optimizations

Virtual DOM Efficiency

Keys: Use keys for list items to enable efficient reordering
Memoization: Cache expensive view computations
Lazy rendering: Defer off-screen content

// Keys for efficient list updates
const moves = game.moves.map((move, i) => 
  h('move', { key: move.id }, renderMove(move))
);

Asset Optimization

Code splitting: Shared chunks cached across pages
Tree shaking: Dead code elimination
Content hashing: Immutable URLs for CDN caching
Lazy loading: Dynamic imports for large features

Browser Compatibility

Browserslist targets modern browsers:

"browserslist": [
  "Firefox >= 115",
  "Chrome >= 112",
  "Safari >= 13.1"
]

No polyfills for older browsers - encourages upgrades for security.

Testing

Unit tests use Node.js test runner:

ui/test                # Run all tests
ui/test -w             # Watch mode
ui/test winning        # Run specific test file

Test files in ui/*/tests/**/*.test.ts

Get Started

Core Features

Development

Architecture

Contributing

Frontend Architecture

Overview

Technology Stack

Core Technologies

Key Libraries

Monorepo Structure

Package Dependencies

Virtual DOM with Snabbdom

Why Snabbdom?

Snabbdom Basics

View Functions

Rendering Cycle

UI Package Architecture

Core Pattern: MVC-style Controllers

Controller Example

Bootstrap Pattern

Shared Library (`ui/lib`)

Chessground Integration

Build System

Package Configuration

Build Properties

Build Commands

Code Splitting

TypeScript Configuration

State Management

Local State Pattern

Communication Between Components

Performance Optimizations

Testing

See Also

Build docs developers (and LLMs) love

Get Started

Core Features

Development

Architecture

Contributing

​Overview

​Technology Stack

​Core Technologies

​Key Libraries

​Monorepo Structure

​Package Dependencies

​Virtual DOM with Snabbdom

​Why Snabbdom?

​Snabbdom Basics

​View Functions

​Rendering Cycle

​UI Package Architecture

​Core Pattern: MVC-style Controllers

​Controller Example

​Bootstrap Pattern

​Shared Library (ui/lib)

​Chessground Integration

​Build System

​Package Configuration

​Build Properties

​Build Commands

​Code Splitting

​TypeScript Configuration

​State Management

​Local State Pattern

​Communication Between Components

​Performance Optimizations

​Testing

​See Also

Build docs developers (and LLMs) love

Overview

Technology Stack

Core Technologies

Key Libraries

Monorepo Structure

Package Dependencies

Virtual DOM with Snabbdom

Why Snabbdom?

Snabbdom Basics

View Functions

Rendering Cycle

UI Package Architecture

Core Pattern: MVC-style Controllers

Controller Example

Bootstrap Pattern

Shared Library (`ui/lib`)

Chessground Integration

Build System

Package Configuration

Build Properties

Build Commands

Code Splitting

TypeScript Configuration

State Management

Local State Pattern

Communication Between Components

Performance Optimizations

Testing

See Also