@chess-openings/eco.json

TypeScript utilities for consuming chess opening data from the eco.json data files.

Features

• 🎯 12,000+ chess openings with ECO codes, move sequences, and names (plus aliases)
• 🔍 FEN-based lookup with automatic position-only fallback
• 🌐 On-demand data loading from GitHub (no bundled files)
• 🔄 Opening transitions with next/previous position navigation
• 📦 TypeScript-first with complete type definitions
• 🚀 Zero dependencies (only needs Node.js 18+)

Installation

npm install @chess-openings/eco.json

Quick Start

import { openingBook, findOpening, getFromTos } from "@chess-openings/eco.json";

// Load all openings
const openings = await openingBook();

// Look up an opening by FEN
const opening = findOpening(
  openings,
  "rnbqkbnr/pppppppp/8/8/4P3/8/PPPP1PPP/RNBQKBNR b KQkq e3 0 1"
);

console.log(opening?.name); // "King's Pawn Opening"
console.log(opening?.eco); // "B00"
console.log(opening?.moves); // "1. e4"

// Get next and previous positions
const { next, from } = await getFromTos(
  "rnbqkbnr/pppppppp/8/8/4P3/8/PPPP1PPP/RNBQKBNR b KQkq e3 0 1"
);

console.log(next[0]?.name); // "Sicilian Defense"
console.log(from[0]?.name); // "Starting Position"

API Reference

Opening Data

getLatestEcoJson(options)

Downloads opening data from GitHub by ECO category.

import { getLatestEcoJson } from "@chess-openings/eco.json";

// Load specific categories
const data = await getLatestEcoJson({
  categories: ["A", "B"],
  includeInterpolated: false,
});

// Load everything (default)
const allData = await getLatestEcoJson();

Options:

• categories?: EcoCategory[] - Array of 'A', 'B', 'C', 'D', 'E' (default: all)
• includeInterpolated?: boolean - Include gap-filling variations (default: true)

openingBook()

Returns a merged collection of all openings.

import { openingBook } from "@chess-openings/eco.json";

const openings = await openingBook();
// OpeningCollection: Record<FEN, Opening>

findOpening(openingBook, fen, positionBook?)

Looks up an opening by FEN with automatic position-only fallback.

import {
  findOpening,
  getPositionBook,
  openingBook,
} from "@chess-openings/eco.json";

const openings = await openingBook();
const posBook = getPositionBook(openings);

const opening = findOpening(openings, fen, posBook);

Parameters:

• openingBook: OpeningCollection - The opening database
• fen: string - FEN string to look up
• positionBook?: PositionBook - Optional position-only mapping for fallback

Returns: Opening | undefined

lookupByMoves(chess, openingBook, options?)

Finds an opening by walking backward through move history. Works with any chess library implementing the ChessGameLike interface (chess.js, @chess-pgn/chess-pgn, etc.).

This function tries to find an opening match at the current position. If no match is found, it undoes moves one by one and tries again, allowing it to find the nearest named opening even when the current position has moved beyond standard opening theory.

import { ChessPGN } from "@chess-pgn/chess-pgn";
import {
  openingBook,
  lookupByMoves,
  getPositionBook,
} from "@chess-openings/eco.json";

const chess = new ChessPGN();
chess.loadPgn("1. e4 e5 2. Nf3 Nc6 3. Bb5 a6 4. Ba4 Nf6");

const openings = await openingBook();
const posBook = getPositionBook(openings);

const result = lookupByMoves(chess, openings, { positionBook: posBook });

console.log(result.opening?.name); // "Ruy Lopez"
console.log(result.movesBack); // 0 (exact match at current position)

Parameters:

• chess: ChessGameLike - Chess game instance (must have fen(), undo(), load() methods)
• openingBook: OpeningCollection - The opening database
• options?: LookupByMovesOptions
  • maxMovesBack?: number - Maximum moves to walk backward (default: 50 plies / move 25)
  • positionBook?: PositionBook - Position book for better matching

Returns: LookupByMovesResult

• opening: Opening | undefined - The opening found (or undefined)
• movesBack: number - Number of moves walked backward (0 = exact match)

ChessGameLike Interface:

Your chess library must implement:

interface ChessGameLike {
  fen(): string; // Returns current FEN
  undo(): unknown; // Undoes last move (returns truthy on success, falsy/throws when no moves)
  load(fen: string): void; // Loads position from FEN
}

Compatible libraries: chess.js, @chess-pgn/chess-pgn, and others with similar APIs.

Example - Position beyond opening theory:

// Many moves deep into a game
chess.loadPgn(
  "1. e4 e5 2. Nf3 Nc6 3. Bb5 a6 4. Ba4 Nf6 5. O-O Be7 6. Re1 b5 7. Bb3 d6 8. c3 O-O 9. h3 Na5"
);

const result = lookupByMoves(chess, openings, { maxMovesBack: 10 });

console.log(result.opening?.name); // "Ruy Lopez: Closed"
console.log(result.movesBack); // 3 (walked back 3 moves to find opening)

getPositionBook(openingBook)

Creates a position-only FEN → full FEN array mapping.

import { getPositionBook } from "@chess-openings/eco.json";

const positionBook = getPositionBook(openings);
// Record<string, string[]>

Query Methods

getOpeningsByEco(ecoCode, openings?)

Returns all openings for a specific ECO code.

import { getOpeningsByEco } from "@chess-openings/eco.json";

// Get all Caro-Kann Defense variations
const caroKann = await getOpeningsByEco("B12");
console.log(`Found ${caroKann.length} variations`);

Parameters:

• ecoCode: ECO code string (e.g., "B12", "C42")
• openings: Optional pre-loaded opening collection

Returns: Promise<Opening[]>

getOpeningsByEcoCategory(category, openings?)

Returns all openings for an ECO category (A, B, C, D, or E).

import {
  getOpeningsByEcoCategory,
  openingBook,
} from "@chess-openings/eco.json";

// Get all semi-open games (category B)
const semiOpen = await getOpeningsByEcoCategory("B");

// More efficient with pre-loaded book
const book = await openingBook();
const catA = await getOpeningsByEcoCategory("A", book);
const catB = await getOpeningsByEcoCategory("B", book);

Parameters:

• category: ECO category letter ("A", "B", "C", "D", or "E")
• openings: Optional pre-loaded opening collection

Returns: Promise<Opening[]>

Throws: Error if category is invalid

getEcoRoots(openings?)

Returns only canonical ECO root variations (where isEcoRoot === true).

import { getEcoRoots } from "@chess-openings/eco.json";

const roots = await getEcoRoots();
console.log(`Found ${Object.keys(roots).length} ECO root variations`);

// Check if a position is an ECO root
const fen = "rnbqkb1r/pppppppp/5n2/8/4P3/8/PPPP1PPP/RNBQKBNR w KQkq - 1 2";
if (roots[fen]) {
  console.log(`${roots[fen].name} is an ECO root`);
}

Parameters:

• openings: Optional pre-loaded opening collection

Returns: Promise<OpeningCollection> (FEN → Opening mapping)

Transition Data

getFromToIndex()

Downloads pre-indexed transition data from GitHub.

import { getFromToIndex } from "@chess-openings/eco.json";

const index = await getFromToIndex();
// { to: Record<string, string[]>, from: Record<string, string[]> }

getFromTos(fen, openings?)

Gets next and previous opening positions with full Opening objects.

import { getFromTos } from "@chess-openings/eco.json";

const { next, from } = await getFromTos(fen);
// { next: Opening[], from: Opening[] }

Parameters:

• fen: string - FEN string to look up transitions for
• openings?: OpeningCollection - Optional pre-loaded opening collection

Returns: { next: Opening[], from: Opening[] }

clearFromToCache()

Clears the cached transition index.

import { clearFromToCache } from "@chess-openings/eco.json";

clearFromToCache();

Types

Opening

interface Opening {
  name: string; // "Sicilian Defense"
  moves: string; // "1. e4 c5"
  eco: string; // "B20"
  score?: number | null; // Position evaluation
  next?: Opening[]; // Next positions
  from?: Opening[]; // Previous positions
  src?: string; // Data source
  isEcoRoot?: boolean; // Root ECO variation
  fen?: string; // FEN string
}

OpeningCollection

type OpeningCollection = Record<FEN, Opening>;

PositionBook

type PositionBook = Record<string, string[]>;

FromToIndex

interface FromToIndex {
  to: Record<string, string[]>; // position → next FENs
  from: Record<string, string[]>; // position → previous FENs
}

Data Structure

The eco.json database contains 12,000+ chess opening variations organized across multiple JSON files:

• ecoA.json - ecoE.json: Main opening databases, one per ECO category (A-E)
• eco_interpolated.json: Generated variations that bridge gaps between named openings
• fromTo.json: Raw transition graph array of [from_fen, to_fen, from_source, to_source]
• fromToPositionIndexed.json: Pre-indexed transition data (optimized for O(1) lookup)

All data is downloaded on-demand from GitHub and cached in memory.

Opening Object Schema

Each opening is keyed by FEN (Forsyth-Edwards Notation), which uniquely identifies positions:

{
  "rnbqkb1r/pppppppp/8/3nP3/3P4/8/PPP2PPP/RNBQKBNR b KQkq": {
    "src": "eco_tsv",
    "eco": "B03",
    "moves": "1. e4 Nf6 2. e5",
    "name": "Alekhine Defense",
    "aliases": {
      "scid": "Alekhine: 3.d4"
    },
    "scid": "B03a",
    "isEcoRoot": true
  }
}

Field Descriptions

• fen (key): Unique position identifier using Forsyth-Edwards Notation. Can be full FEN (with turn, castling, en passant) or position-only (first field).
• src: Data source - eco_tsv (authoritative), eco_js, scid, eco_wikip, wiki_b, ct, chessGraph, chronos, icsbot, pgn, or interpolated
• eco: ECO category code (e.g., "B03") - not unique; many openings share the same ECO code
• moves: Standard algebraic notation move sequence (e.g., "1. e4 Nf6 2. e5")
• name: Opening name (e.g., "Alekhine Defense")
• aliases: Alternative names from other sources (object with source keys)
• scid: Extended SCID code when applicable (e.g., "B03a")
• isEcoRoot: true if this is the canonical root variation for this ECO code

Source Hierarchy

eco_tsv (from lichess) is the authoritative source and supersedes all conflicts. Other sources are merged with eco_tsv taking precedence.

Interpolated Openings

"Orphan" variations lack a from field (no preceding named variation). Interpolations fill these gaps:

{
  "rnbqkbnr/pppppppp/8/4P3/3P4/8/PPP2PPP/RNBQKBNR b KQkq - 0 3": {
    "src": "interpolated",
    "eco": "B02",
    "moves": "1. e4 Nf6 2. e5 Ng8 3. d4",
    "name": "Alekhine Defense: Brooklyn Variation (i)",
    "scid": "B02l",
    "aliases": {
      "scid": "Alekhine: Brooklyn Defence (Retreat Variation)"
    }
  }
}

Key characteristics:

• src: "interpolated"
• name: Root name + " (i)" suffix
• Purpose: Every move subsequence that appears within an opening has an entry

For example, if "Queen's Gambit Declined: Exchange Variation" is "1. d4 Nf6 2. c4 e6 3. Nc3 d5 4. cxd5" but no entry exists for "1. d4 Nf6 2. c4 e6 3. Nc3 d5", an interpolated opening is created to bridge the gap.

FEN as Primary Key

• FEN strings uniquely identify positions
• Move sequences and names are NOT unique identifiers
• Multiple move orders (transpositions) can reach the same FEN
• Always use FEN for lookups, not moves or names

Data Sources

This data is a collation of several chess opening databases:

• eco_tsv: lichess chess-openings (Jun. 2025)
• eco_js: Original eco.json data from several years ago
• scid: SCID project database
• eco_wikip: Wikipedia List of Chess Openings (Aug. 2024)
• wiki_b: Wikibooks Chess Opening Theory (Nov. 2024)
• ct: ChessTempo (Jun 2025)
• chessGraph: chess-graph (Jun 2025)
• chronos: chronos eco.pgn (Jun 2025)
• icsbot: icsbot (Jun 2025)
• pgn: Various PGN file databases (Jun 2025)

Performance Tips

1. Reuse loaded data: Call openingBook() once and reuse the result
2. Pre-build position book: Create once with getPositionBook() if doing multiple lookups
3. Cache transitions: getFromTos() caches the index automatically
4. Pass openings: Provide pre-loaded openings to getFromTos() to skip re-fetching

// Efficient pattern
const openings = await openingBook();
const posBook = getPositionBook(openings);

for (const fen of fens) {
  const opening = findOpening(openings, fen, posBook);
  const transitions = await getFromTos(fen, openings);
}

Acknowledgements

Thanks to @niklasf for eco.

Credit goes to Shane Hudson for the original SCID opening data.

Original eco.json data was compiled by Ömür Yanıkoğlu.

Contributing

This package provides read-only utilities for consuming opening data. For data maintenance and updates:

• Data source: eco.json repository
• Data tooling: eco.json.tooling repository

License

MIT

Related Projects

• eco.json - Chess opening database
• fensterchess - Chess opening research tool