chess.js

chess.js is a Javascript chess library that is used for chess move generation/validation, piece placement/movement, and check/checkmate/stalemate detection - basically everything but the AI.

chess.js has been extensively tested in node.js and most modern browsers.

Installation

To install the stable version:

# NPM
npm install chess.js

# Yarn
yarn add chess.js

Example Code

The code below plays a random game of chess:

const { Chess } = require('./chess.js')
const chess = new Chess()

while (!chess.game_over()) {
    const moves = chess.moves()
    const move = moves[Math.floor(Math.random() * moves.length)]
    chess.move(move)
}
console.log(chess.pgn())

User Interface

By design, chess.js is headless and does not include user interface. Many developers have had success integrating chess.js with the chessboard.js library. See chessboard.js - Random vs Random for an example.

API

Constructor: Chess([ fen ])

The Chess() constructor takes an optional parameter which specifies the board configuration in Forsyth-Edwards Notation.

// board defaults to the starting position when called with no parameters
const chess = new Chess()

// pass in a FEN string to load a particular position
const chess = new Chess(
    'r1k4r/p2nb1p1/2b4p/1p1n1p2/2PP4/3Q1NB1/1P3PPP/R5K1 b - c3 0 19'
)

.ascii()

Returns a string containing an ASCII diagram of the current position.

const chess = new Chess()

// make some moves
chess.move('e4')
chess.move('e5')
chess.move('f4')

chess.ascii()
// -> '   +------------------------+
//      8 | r  n  b  q  k  b  n  r |
//      7 | p  p  p  p  .  p  p  p |
//      6 | .  .  .  .  .  .  .  . |
//      5 | .  .  .  .  p  .  .  . |
//      4 | .  .  .  .  P  P  .  . |
//      3 | .  .  .  .  .  .  .  . |
//      2 | P  P  P  P  .  .  P  P |
//      1 | R  N  B  Q  K  B  N  R |
//        +------------------------+
//          a  b  c  d  e  f  g  h'

.board()

Returns an 2D array representation of the current position. Empty squares are represented by null.

const chess = new Chess()

chess.board()
// -> [[{type: 'r', color: 'b'},
        {type: 'n', color: 'b'},
        {type: 'b', color: 'b'},
        {type: 'q', color: 'b'},
        {type: 'k', color: 'b'},
        {type: 'b', color: 'b'},
        {type: 'n', color: 'b'},
        {type: 'r', color: 'b'}],
        [...],
        [...],
        [...],
        [...],
        [...],
        [{type: 'r', color: 'w'},
         {type: 'n', color: 'w'},
         {type: 'b', color: 'w'},
         {type: 'q', color: 'w'},
         {type: 'k', color: 'w'},
         {type: 'b', color: 'w'},
         {type: 'n', color: 'w'},
         {type: 'r', color: 'w'}]]

.clear()

Clears the board.

chess.clear()
chess.fen()
// -> '8/8/8/8/8/8/8/8 w - - 0 1' <- empty board

.delete_comment()

Delete and return the comment for the current position, if it exists.

const chess = new Chess()

chess.load_pgn("1. e4 e5 2. Nf3 Nc6 3. Bc4 Bc5 {giuoco piano} *")

chess.get_comment()
// -> "giuoco piano"

chess.delete_comment()
// -> "giuoco piano"

chess.get_comment()
// -> undefined

.delete_comments()

Delete and return comments for all positions.

const chess = new Chess()

chess.load_pgn("1. e4 e5 {king's pawn opening} 2. Nf3 Nc6 3. Bc4 Bc5 {giuoco piano} *")

chess.delete_comments()
// -> [
//     {
//       fen: "rnbqkbnr/pppp1ppp/8/4p3/4P3/8/PPPP1PPP/RNBQKBNR w KQkq e6 0 2",
//       comment: "king's pawn opening"
//     },
//     {
//       fen: "r1bqkbnr/pppp1ppp/2n5/4p3/4P3/5N2/PPPP1PPP/RNBQKB1R w KQkq - 2 3",
//       comment: "giuoco piano"
//     }
//    ]

chess.get_comments()
// -> []

.fen()

Returns the FEN string for the current position.

const chess = new Chess()

// make some moves
chess.move('e4')
chess.move('e5')
chess.move('f4')

chess.fen()
// -> 'rnbqkbnr/pppp1ppp/8/4p3/4PP2/8/PPPP2PP/RNBQKBNR b KQkq f3 0 2'

.game_over()

Returns true if the game has ended via checkmate, stalemate, draw, threefold repetition, or insufficient material. Otherwise, returns false.

const chess = new Chess()
chess.game_over()
// -> false

// stalemate
chess.load('4k3/4P3/4K3/8/8/8/8/8 b - - 0 78')
chess.game_over()
// -> true

// checkmate
chess.load('rnb1kbnr/pppp1ppp/8/4p3/5PPq/8/PPPPP2P/RNBQKBNR w KQkq - 1 3')
chess.game_over()
// -> true

.get(square)

Returns the piece on the square:

chess.clear()
chess.put({ type: chess.PAWN, color: chess.BLACK }, 'a5') // put a black pawn on a5

chess.get('a5')
// -> { type: 'p', color: 'b' },
chess.get('a6')
// -> null

.get_comment()

Retrieve the comment for the current position, if it exists.

const chess = new Chess()

chess.load_pgn("1. e4 e5 2. Nf3 Nc6 3. Bc4 Bc5 {giuoco piano} *")

chess.get_comment()
// -> "giuoco piano"

.get_comments()

Retrieve comments for all positions.

const chess = new Chess()

chess.load_pgn("1. e4 e5 {king's pawn opening} 2. Nf3 Nc6 3. Bc4 Bc5 {giuoco piano} *")

chess.get_comments()
// -> [
//     {
//       fen: "rnbqkbnr/pppp1ppp/8/4p3/4P3/8/PPPP1PPP/RNBQKBNR w KQkq e6 0 2",
//       comment: "king's pawn opening"
//     },
//     {
//       fen: "r1bqkbnr/pppp1ppp/2n5/4p3/4P3/5N2/PPPP1PPP/RNBQKB1R w KQkq - 2 3",
//       comment: "giuoco piano"
//     }
//    ]

.header()

Allows header information to be added to PGN output. Any number of key/value pairs can be passed to .header().

chess.header('White', 'Robert James Fischer')
chess.header('Black', 'Mikhail Tal')

// or

chess.header('White', 'Morphy', 'Black', 'Anderssen', 'Date', '1858-??-??')

Calling .header() without any arguments returns the header information as an object.

chess.header()
// -> { White: 'Morphy', Black: 'Anderssen', Date: '1858-??-??' }

.history([ options ])

Returns a list containing the moves of the current game. Options is an optional parameter which may contain a 'verbose' flag. See .moves() for a description of the verbose move fields.

const chess = new Chess()
chess.move('e4')
chess.move('e5')
chess.move('f4')
chess.move('exf4')

chess.history()
// -> ['e4', 'e5', 'f4', 'exf4']

chess.history({ verbose: true })
// -> [{ color: 'w', from: 'e2', to: 'e4', flags: 'b', piece: 'p', san: 'e4' },
//     { color: 'b', from: 'e7', to: 'e5', flags: 'b', piece: 'p', san: 'e5' },
//     { color: 'w', from: 'f2', to: 'f4', flags: 'b', piece: 'p', san: 'f4' },
//     { color: 'b', from: 'e5', to: 'f4', flags: 'c', piece: 'p', captured: 'p', san: 'exf4' }]

.in_check()

Returns true or false if the side to move is in check.

const chess = new Chess(
    'rnb1kbnr/pppp1ppp/8/4p3/5PPq/8/PPPPP2P/RNBQKBNR w KQkq - 1 3'
)
chess.in_check()
// -> true

.in_checkmate()

Returns true or false if the side to move has been checkmated.

const chess = new Chess(
    'rnb1kbnr/pppp1ppp/8/4p3/5PPq/8/PPPPP2P/RNBQKBNR w KQkq - 1 3'
)
chess.in_checkmate()
// -> true

.in_draw()

Returns true or false if the game is drawn (50-move rule or insufficient material).

const chess = new Chess('4k3/4P3/4K3/8/8/8/8/8 b - - 0 78')
chess.in_draw()
// -> true

.in_stalemate()

Returns true or false if the side to move has been stalemated.

const chess = new Chess('4k3/4P3/4K3/8/8/8/8/8 b - - 0 78')
chess.in_stalemate()
// -> true

.in_threefold_repetition()

Returns true or false if the current board position has occurred three or more times.

const chess = new Chess('rnbqkbnr/pppppppp/8/8/8/8/PPPPPPPP/RNBQKBNR w KQkq - 0 1')
// -> true
// rnbqkbnr/pppppppp/8/8/8/8/PPPPPPPP/RNBQKBNR w KQkq occurs 1st time
chess.in_threefold_repetition()
// -> false

chess.move('Nf3') chess.move('Nf6') chess.move('Ng1') chess.move('Ng8')
// rnbqkbnr/pppppppp/8/8/8/8/PPPPPPPP/RNBQKBNR w KQkq occurs 2nd time
chess.in_threefold_repetition()
// -> false

chess.move('Nf3') chess.move('Nf6') chess.move('Ng1') chess.move('Ng8')
// rnbqkbnr/pppppppp/8/8/8/8/PPPPPPPP/RNBQKBNR w KQkq occurs 3rd time
chess.in_threefold_repetition()
// -> true

.insufficient_material()

Returns true if the game is drawn due to insufficient material (K vs. K, K vs. KB, or K vs. KN) otherwise false.

const chess = new Chess('k7/8/n7/8/8/8/8/7K b - - 0 1')
chess.insufficient_material()
// -> true

.load(fen)

The board is cleared, and the FEN string is loaded. Returns true if the position was successfully loaded, otherwise false.

const chess = new Chess()
chess.load('4r3/8/2p2PPk/1p6/pP2p1R1/P1B5/2P2K2/3r4 w - - 1 45')
// -> true

chess.load('4r3/8/X12XPk/1p6/pP2p1R1/P1B5/2P2K2/3r4 w - - 1 45')
// -> false, bad piece X

.load_pgn(pgn, [ options ])

Load the moves of a game stored in Portable Game Notation. pgn should be a string. Options is an optional object which may contain a string newline_char and a boolean sloppy.

The newline_char is a string representation of a valid RegExp fragment and is used to process the PGN. It defaults to \r?\n. Special characters should not be pre-escaped, but any literal special characters should be escaped as is normal for a RegExp. Keep in mind that backslashes in JavaScript strings must themselves be escaped (see sloppy_pgn example below). Avoid using a newline_char that may occur elsewhere in a PGN, such as . or x, as this will result in unexpected behavior.

The sloppy flag is a boolean that permits chess.js to parse moves in non-standard notations. See .move documentation for more information about non-SAN notations.

The method will return true if the PGN was parsed successfully, otherwise false.

const chess = new Chess()
const pgn = [
    '[Event "Casual Game"]',
    '[Site "Berlin GER"]',
    '[Date "1852.??.??"]',
    '[EventDate "?"]',
    '[Round "?"]',
    '[Result "1-0"]',
    '[White "Adolf Anderssen"]',
    '[Black "Jean Dufresne"]',
    '[ECO "C52"]',
    '[WhiteElo "?"]',
    '[BlackElo "?"]',
    '[PlyCount "47"]',
    '',
    '1.e4 e5 2.Nf3 Nc6 3.Bc4 Bc5 4.b4 Bxb4 5.c3 Ba5 6.d4 exd4 7.O-O',
    'd3 8.Qb3 Qf6 9.e5 Qg6 10.Re1 Nge7 11.Ba3 b5 12.Qxb5 Rb8 13.Qa4',
    'Bb6 14.Nbd2 Bb7 15.Ne4 Qf5 16.Bxd3 Qh5 17.Nf6+ gxf6 18.exf6',
    'Rg8 19.Rad1 Qxf3 20.Rxe7+ Nxe7 21.Qxd7+ Kxd7 22.Bf5+ Ke8',
    '23.Bd7+ Kf8 24.Bxe7# 1-0'
]

chess.load_pgn(pgn.join('\n'))
// -> true

chess.fen()
// -> 1r3kr1/pbpBBp1p/1b3P2/8/8/2P2q2/P4PPP/3R2K1 b - - 0 24

chess.ascii()
// -> '  +------------------------+
//     8 | .  r  .  .  .  k  r  . |
//     7 | p  b  p  B  B  p  .  p |
//     6 | .  b  .  .  .  P  .  . |
//     5 | .  .  .  .  .  .  .  . |
//     4 | .  .  .  .  .  .  .  . |
//     3 | .  .  P  .  .  q  .  . |
//     2 | P  .  .  .  .  P  P  P |
//     1 | .  .  .  R  .  .  K  . |
//       +------------------------+
//         a  b  c  d  e  f  g  h'

// Parse non-standard move formats and unusual line separators
const sloppy_pgn = [
    '[Event "Wijk aan Zee (Netherlands)"]',
    '[Date "1971.01.26"]',
    '[Result "1-0"]',
    '[White "Tigran Vartanovich Petrosian"]',
    '[Black "Hans Ree"]',
    '[ECO "A29"]',
    '',
    '1. Pc2c4 Pe7e5', // non-standard
    '2. Nc3 Nf6',
    '3. Nf3 Nc6',
    '4. g2g3 Bb4', // non-standard
    '5. Nd5 Nxd5',
    '6. c4xd5 e5-e4', // non-standard
    '7. dxc6 exf3',
    '8. Qb3 1-0'
].join('|')

const options = {
    newline_char: '\\|', // Literal '|' character escaped
    sloppy: true
}

chess.load_pgn(sloppy_pgn)
// -> false

chess.load_pgn(sloppy_pgn, options)
// -> true

chess.fen()
// -> 'r1bqk2r/pppp1ppp/2P5/8/1b6/1Q3pP1/PP1PPP1P/R1B1KB1R b KQkq - 1 8'

.move(move, [ options ])

Attempts to make a move on the board, returning a move object if the move was legal, otherwise null. The .move function can be called two ways, by passing a string in Standard Algebraic Notation (SAN):

const chess = new Chess()

chess.move('e4')
// -> { color: 'w', from: 'e2', to: 'e4', flags: 'b', piece: 'p', san: 'e4' }

chess.move('nf6') // SAN is case sensitive!!
// -> null

chess.move('Nf6')
// -> { color: 'b', from: 'g8', to: 'f6', flags: 'n', piece: 'n', san: 'Nf6' }

Or by passing .move() a move object (only the 'to', 'from', and when necessary 'promotion', fields are needed):

const chess = new Chess()

chess.move({ from: 'g2', to: 'g3' })
// -> { color: 'w', from: 'g2', to: 'g3', flags: 'n', piece: 'p', san: 'g3' }

An optional sloppy flag can be used to parse a variety of non-standard move notations:

const chess = new Chess()

// various forms of Long Algebraic Notation
chess.move('e2e4', { sloppy: true })
// -> { color: 'w', from: 'e2', to: 'e4', flags: 'b', piece: 'p', san: 'e4' }
chess.move('e7-e5', { sloppy: true })
// -> { color: 'b', from: 'e7', to: 'e5', flags: 'b', piece: 'p', san: 'e5' }
chess.move('Pf2f4', { sloppy: true })
// -> { color: 'w', from: 'f2', to: 'f4', flags: 'b', piece: 'p', san: 'f4' }
chess.move('Pe5xf4', { sloppy: true })
// -> { color: 'b', from: 'e5', to: 'f4', flags: 'c', piece: 'p', captured: 'p', san: 'exf4' }

// correctly parses incorrectly disambiguated moves
chess = new Chess(
    'r2qkbnr/ppp2ppp/2n5/1B2pQ2/4P3/8/PPP2PPP/RNB1K2R b KQkq - 3 7'
)

chess.move('Nge7') // Ne7 is unambiguous because the knight on c6 is pinned
// -> null

chess.move('Nge7', { sloppy: true })
// -> { color: 'b', from: 'g8', to: 'e7', flags: 'n', piece: 'n', san: 'Ne7' }

.moves([ options ])

Returns a list of legal moves from the current position. The function takes an optional parameter which controls the single-square move generation and verbosity.

const chess = new Chess()
chess.moves()
// -> ['a3', 'a4', 'b3', 'b4', 'c3', 'c4', 'd3', 'd4', 'e3', 'e4',
//     'f3', 'f4', 'g3', 'g4', 'h3', 'h4', 'Na3', 'Nc3', 'Nf3', 'Nh3']

chess.moves({ square: 'e2' })
// -> ['e3', 'e4']

chess.moves({ square: 'e9' }) // invalid square
// -> []

chess.moves({ verbose: true })
// -> [{ color: 'w', from: 'a2', to: 'a3',
//       flags: 'n', piece: 'p', san 'a3'
//       # a captured: key is included when the move is a capture
//       # a promotion: key is included when the move is a promotion
//     },
//     ...
//     ]

The piece, captured, and promotion fields contain the lowercase representation of the applicable piece.

The flags field in verbose mode may contain one or more of the following values:

• 'n' - a non-capture
• 'b' - a pawn push of two squares
• 'e' - an en passant capture
• 'c' - a standard capture
• 'p' - a promotion
• 'k' - kingside castling
• 'q' - queenside castling

A flag of 'pc' would mean that a pawn captured a piece on the 8th rank and promoted.

.pgn([ options ])

Returns the game in PGN format. Options is an optional parameter which may include max width and/or a newline character settings.

const chess = new Chess()
chess.header('White', 'Plunky', 'Black', 'Plinkie')
chess.move('e4')
chess.move('e5')
chess.move('Nc3')
chess.move('Nc6')

chess.pgn({ max_width: 5, newline_char: '<br />' })
// -> '[White "Plunky"]<br />[Black "Plinkie"]<br /><br />1. e4 e5<br />2. Nc3 Nc6'

.put(piece, square)

Place a piece on the square where piece is an object with the form { type: ..., color: ... }. Returns true if the piece was successfully placed, otherwise, the board remains unchanged and false is returned. put() will fail when passed an invalid piece or square, or when two or more kings of the same color are placed.

chess.clear()

chess.put({ type: chess.PAWN, color: chess.BLACK }, 'a5') // put a black pawn on a5
// -> true
chess.put({ type: 'k', color: 'w' }, 'h1') // shorthand
// -> true

chess.fen()
// -> '8/8/8/p7/8/8/8/7K w - - 0 0'

chess.put({ type: 'z', color: 'w' }, 'a1') // invalid piece
// -> false

chess.clear()

chess.put({ type: 'k', color: 'w' }, 'a1')
// -> true

chess.put({ type: 'k', color: 'w' }, 'h1') // fail - two kings
// -> false

.remove(square)

Remove and return the piece on square.

chess.clear()
chess.put({ type: chess.PAWN, color: chess.BLACK }, 'a5') // put a black pawn on a5
chess.put({ type: chess.KING, color: chess.WHITE }, 'h1') // put a white king on h1

chess.remove('a5')
// -> { type: 'p', color: 'b' },
chess.remove('h1')
// -> { type: 'k', color: 'w' },
chess.remove('e1')
// -> null

.reset()

Reset the board to the initial starting position.

.set_comment(comment)

Comment on the current position.

const chess = new Chess()

chess.move("e4")
chess.set_comment("king's pawn opening")

chess.pgn()
// -> "1. e4 {king's pawn opening}"

.square_color(square)

Returns the color of the square ('light' or 'dark').

const chess = Chess()
chess.square_color('h1')
// -> 'light'
chess.square_color('a7')
// -> 'dark'
chess.square_color('bogus square')
// -> null

.turn()

Returns the current side to move.

chess.load('rnbqkbnr/pppppppp/8/8/4P3/8/PPPP1PPP/RNBQKBNR b KQkq e3 0 1')
chess.turn()
// -> 'b'

.undo()

Takeback the last half-move, returning a move object if successful, otherwise null.

const chess = new Chess()

chess.fen()
// -> 'rnbqkbnr/pppppppp/8/8/8/8/PPPPPPPP/RNBQKBNR w KQkq - 0 1'
chess.move('e4')
chess.fen()
// -> 'rnbqkbnr/pppppppp/8/8/4P3/8/PPPP1PPP/RNBQKBNR b KQkq e3 0 1'

chess.undo()
// -> { color: 'w', from: 'e2', to: 'e4', flags: 'b', piece: 'p', san: 'e4' }
chess.fen()
// -> 'rnbqkbnr/pppppppp/8/8/8/8/PPPPPPPP/RNBQKBNR w KQkq - 0 1'
chess.undo()
// -> null

.validate_fen(fen):

Returns a validation object specifying validity or the errors found within the FEN string.

chess.validate_fen('2n1r3/p1k2pp1/B1p3b1/P7/5bP1/2N1B3/1P2KP2/2R5 b - - 4 25')
// -> { valid: true, error_number: 0, error: 'No errors.' }

chess.validate_fen('4r3/8/X12XPk/1p6/pP2p1R1/P1B5/2P2K2/3r4 w - - 1 45')
// -> { valid: false, error_number: 9,
//     error: '1st field (piece positions) is invalid [invalid piece].' }

MUSIC

Musical support provided by:

• The Grateful Dead
• Umphrey's McGee

BUGS

• The en passant square and castling flags aren't adjusted when using the put/remove functions (workaround: use .load() instead)