chess.js

chess.js is a TypeScript chess library 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

Run the following command to install the most recent version of chess.js from NPM:

npm install chess.js

Importing

Import (as ESM)

import { Chess } from 'chess.js'

ECMAScript modules (ESM) can be directly imported in a browser:

Import (as CommonJS)

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

Example Code

The code below plays a random game of chess:

import { Chess } from 'chess.js'

const chess = new Chess()

while (!chess.isGameOver()) {
  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 a headless library and does not include user interface elements. Many developers have successfully integrated chess.js with the chessboard.js library. See chessboard.js - Random vs Random for an example.

Parsers (permissive / strict)

This library includes two parsers (permissive and strict) which are used to parse different forms of chess move notation. The permissive parser (the default) is able to handle many non-standard derivatives of algebraic notation (e.g. Nf3, g1f3, g1-f3, Ng1f3, Ng1-f3, Ng1xf3). The strict parser only accepts moves in Standard Algebraic Notation and requires that they strictly adhere to the specification. The strict parser runs slightly faster but will not parse any non-standard notation.

API

Constants

The following constants are exported from the top-level module:

// colors
export const WHITE = 'w'
export const BLACK = 'b'

// pieces
export const PAWN = 'p'
export const KNIGHT = 'n'
export const BISHOP = 'b'
export const ROOK = 'r'
export const QUEEN = 'q'
export const KING = 'k'

// starting position (in FEN)
export const DEFAULT_POSITION = 'rnbqkbnr/pppppppp/8/8/8/8/PPPPPPPP/RNBQKBNR w KQkq - 0 1'

// square list
export const SQUARES = ['a8', 'b8', 'c8', ..., 'f1', 'g1', 'h1']

Constructor: Chess([ fen ])

The Chess() constructor takes an optional parameter which specifies the board configuration in Forsyth-Edwards Notation (FEN). Throws an exception if an invalid FEN string is provided.

import { Chess } from 'chess.js'

// an empty constructor defaults the starting position
let chess = new Chess()

// pass in a FEN string to load a particular position
let chess = new Chess(
  'r1k4r/p2nb1p1/2b4p/1p1n1p2/2PP4/3Q1NB1/1P3PPP/R5K1 b - - 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()
// -> [[{square: 'a8', type: 'r', color: 'b'},
        {square: 'b8', type: 'n', color: 'b'},
        {square: 'c8', type: 'b', color: 'b'},
        {square: 'd8', type: 'q', color: 'b'},
        {square: 'e8', type: 'k', color: 'b'},
        {square: 'f8', type: 'b', color: 'b'},
        {square: 'g8', type: 'n', color: 'b'},
        {square: 'h8', type: 'r', color: 'b'}],
        [...],
        [...],
        [...],
        [...],
        [...],
        [{square: 'a1', type: 'r', color: 'w'},
         {square: 'b1', type: 'n', color: 'w'},
         {square: 'c1', type: 'b', color: 'w'},
         {square: 'd1', type: 'q', color: 'w'},
         {square: 'e1', type: 'k', color: 'w'},
         {square: 'f1', type: 'b', color: 'w'},
         {square: 'g1', type: 'n', color: 'w'},
         {square: 'h1', type: 'r', color: 'w'}]]

.clear({ preserveHeaders = false } = {})

Clears the board.

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

.deleteComment()

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

const chess = new Chess()

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

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

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

chess.getComment()
// -> undefined

.deleteComments()

Delete and return comments for all positions.

const chess = new Chess()

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

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

chess.getComments()
// -> []

.fen()

Returns the FEN string for the current position. Note, the en passant square is only included if the side-to-move can legally capture en passant.

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 - 0 2'

.get(square)

Returns the piece on the square:

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

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

.getCastlingRights(color)

Gets the castling rights for the given color. An object is returned which indicates whether the right is available or not for both kingside and queenside. Note this does not indicate if such a move is legal or not in the current position as checks etc. also need to be considered.

const chess = new Chess()

chess.getCastlingRights(BLACK) // black can castle queenside only
// -> { 'k': false, 'q': true }

.getComment()

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

const chess = new Chess()

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

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

.getComments()

Retrieve comments for all positions.

const chess = new Chess()

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

chess.getComments()
// -> [
//     {
//       fen: "rnbqkbnr/pppp1ppp/8/4p3/4P3/8/PPPP1PPP/RNBQKBNR w KQkq - 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. A FEN string of the position prior to the move being made is added to the verbose history output.

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 })
// -->
// [
//   {
//     before: 'rnbqkbnr/pppppppp/8/8/8/8/PPPPPPPP/RNBQKBNR w KQkq - 0 1',
//     after: 'rnbqkbnr/pppppppp/8/8/4P3/8/PPPP1PPP/RNBQKBNR b KQkq - 0 1',
//     color: 'w',
//     piece: 'p',
//     from: 'e2',
//     to: 'e4',
//     san: 'e4',
//     lan: 'e2e4',
//     flags: 'b'
//   },
//   {
//     before: 'rnbqkbnr/pppppppp/8/8/4P3/8/PPPP1PPP/RNBQKBNR b KQkq - 0 1',
//     after: 'rnbqkbnr/pppp1ppp/8/4p3/4P3/8/PPPP1PPP/RNBQKBNR w KQkq - 0 2',
//     color: 'b',
//     piece: 'p',
//     from: 'e7',
//     to: 'e5',
//     san: 'e5',
//     lan: 'e7e5',
//     flags: 'b'
//   },
//   {
//     before: 'rnbqkbnr/pppp1ppp/8/4p3/4P3/8/PPPP1PPP/RNBQKBNR w KQkq - 0 2',
//     after: 'rnbqkbnr/pppp1ppp/8/4p3/4PP2/8/PPPP2PP/RNBQKBNR b KQkq - 0 2',
//     color: 'w',
//     piece: 'p',
//     from: 'f2',
//     to: 'f4',
//     san: 'f4',
//     lan: 'f2f4',
//     flags: 'b'
//   },
//   {
//     before: 'rnbqkbnr/pppp1ppp/8/4p3/4PP2/8/PPPP2PP/RNBQKBNR b KQkq - 0 2',
//     after: 'rnbqkbnr/pppp1ppp/8/8/4Pp2/8/PPPP2PP/RNBQKBNR w KQkq - 0 3',
//     color: 'b',
//     piece: 'p',
//     from: 'e5',
//     to: 'f4',
//     san: 'exf4',
//     lan: 'e5f4',
//     flags: 'c',
//     captured: 'p'
//   }
// ]

.inCheck()

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.inCheck()
// -> true

.isAttacked(square, color)

Returns true if the square is attacked by any piece of the given color.

const chess = new Chess()
chess.isAttacked('f3', WHITE)
// -> true (we can attack empty squares)

chess.isAttacked('f6', BLACK)
// -> true (side to move (e.g. the value returned by .turn) is ignored)

chess.load(DEFAULT_POSITION)
chess.isAttacked('e2', WHITE)
// -> true (we can attack our own pieces)

chess.load('4k3/4n3/8/8/8/8/4R3/4K3 w - - 0 1')
chess.isAttacked('c6', BLACK)
// -> true (pieces still attack a square if even they are pinned)

.isCheckmate()

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.isCheckmate()
// -> true

.isDraw()

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.isDraw()
// -> true

.isInsufficientMaterial()

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.isInsufficientMaterial()
// -> true

.isGameOver()

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

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

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

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

.isStalemate()

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.isStalemate()
// -> true

.isThreefoldRepetition()

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.isThreefoldRepetition()
// -> 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.isThreefoldRepetition()
// -> 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.isThreefoldRepetition()
// -> true

.load(fen: string, { skipValidation = false, preserveHeaders = false } = {})

Clears the board and loads the provided FEN string. The castling rights, en passant square and move numbers are defaulted to - - 0 1 if omitted. Throws an exception if the FEN is invalid.

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

try {
  chess.load('8/4p3/8/8/8/8/4P3/6K1 w - - 1 45')
} catch (e) {
  console.log(e)
}
// -> Error: Invalid FEN: missing black king

chess.load('8/4p3/8/8/8/8/4P3/6K1 w - - 1 45', { skipValidation: true })
// -> Works!

.loadPgn(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 newlineChar and a boolean strict.

The newlineChar 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 sloppyPgn example below). Avoid using a newlineChar that may occur elsewhere in a PGN, such as . or x, as this will result in unexpected behavior.

The strict flag is a boolean (default: false) that instructs chess.js to only parse moves in Standard Algebraic Notation form. See .move documentation for more information about non-SAN notations.

The method will throw and exception if the PGN fails to parse.

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.loadPgn(pgn.join('\n'))

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 sloppyPgn = [
  '[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(':')

chess.loadPgn(sloppyPgn, { newlineChar: ':' })
// works by default

chess.loadPgn(sloppyPgn, { newlineChar: ':', strict: true })
// Error: Invalid move in PGN: Pc2c4

.move(move, [ options ])

Makes a move on the board and returns a move object if the move was legal. The move argument can be either a string in Standard Algebraic Notation (SAN) or a move object. Throws an 'Illegal move' exception if the move was illegal.

.move() - 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!!
// Error: Invalid move: nf6

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

.move() - Object Notation

A move object contains to, from and, promotion (only when necessary) fields.

const chess = new Chess()

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

.move() - Permissive Parser

The permissive (default) move parser can be used to parse a variety of non-standard move notations. Users may specify an { strict: true } flag to verify that all supplied moves adhere to the Standard Algebraic Notation specification.

const chess = new Chess()

// permissive parser accepts various forms of algebraic notation
chess.move('e2e4')
chess.move('e7-e5')
chess.move('Pf2-f4')
chess.move('ef4') // missing 'x' in capture
chess.move('Ng1-f3')
chess.move('d7xd6') // ignore 'x' when not a capture
chess.move('d4')

// correctly parses incorrectly disambiguated moves
chess.load('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
chess.undo()
chess.move('Nge7', { strict: true }) // strict SAN requires Ne7
// Error: Invalid move: Nge7

.moveNumber()

Returns the current move number.

chess.load('4r1k1/p1prnpb1/Pp1pq1pp/3Np2P/2P1P3/R4N2/1PP2PP1/3QR1K1 w - - 2 20')
chess.moveNumber()
// -> 20

.moves({ piece?: Piece, square?: Square, verbose = false} = {})

Returns a list of legal moves from the current position. This function takes an optional object which can be used to generate detailed move objects or to restrict the move generator to specific squares or pieces.

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' }) // single square move generation
// -> ['e3', 'e4']

chess.moves({ piece: 'n' }) // generate moves for piece type
// ['Na3', 'Nc3', 'Nf3', 'Nh3']

chess.moves({ verbose: true }) // return verbose moves
// -> [{ color: 'w', from: 'a2', to: 'a3',
//       flags: 'n', piece: 'p',
//       san 'a3', 'lan', 'a2a3',
//       before: 'rnbqkbnr/pppppppp/8/8/8/8/PPPPPPPP/RNBQKBNR w KQkq - 0 1'
//       after: 'rnbqkbnr/pppppppp/8/8/8/P7/1PPPPPPP/RNBQKBNR b KQkq - 0 1'
//       # a `captured` field is included when the move is a capture
//       # a `promotion` field is included when the move is a promotion
//     },
//     ...
//     ]

Move Objects (e.g. when { verbose: true })

The color field indicates the color of the moving piece (w or b).

The from and to fields are from and to squares in algebraic notation.

The piece, captured, and promotion fields contain the lowercase representation of the applicable piece (pnbrqk). The captured and promotion fields are only present when the move is a valid capture or promotion.

The san field is the move in Standard Algebraic Notation (SAN). The lan field is the move in Long Algebraic Notation (LAN).

The before and after keys contain the FEN of the position before and after the move.

The flags field contains one or more of the string 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 flags value 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({ maxWidth: 5, newline: '<br>' })
// -&gt; '[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: PAWN, color: 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: PAWN, color: BLACK }, 'a5') // put a black pawn on a5
chess.put({ type: KING, color: 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.

.setCastlingRights(color, rights)

Sets the castling rights for the given color. Returns true if the change was successfully made. False will be returned when the position doesn't allow the requested change i.e. if the corresponding king or rook is not on it's starting square.

// white can't castle kingside but can castle queenside
chess.setCastlingRights(WHITE, { [KING]: false, [QUEEN]: true })

.setComment(comment)

Comment on the current position.

const chess = new Chess()

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

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

.squareColor(square)

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

const chess = Chess()
chess.squareColor('h1')
// -> 'light'
chess.squareColor('a7')
// -> 'dark'
chess.squareColor('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 - 0 1'

chess.undo()
//  {
//    color: 'w',
//    piece: 'p',
//    from: 'e2',
//    to: 'e4',
//    san: 'e4',
//    flags: 'b',
//    lan: 'e2e4',
//    before: 'rnbqkbnr/pppppppp/8/8/8/8/PPPPPPPP/RNBQKBNR w KQkq - 0 1',
//    after: 'rnbqkbnr/pppppppp/8/8/4P3/8/PPPP1PPP/RNBQKBNR b KQkq - 0 1'
//  }

chess.fen()
// -> 'rnbqkbnr/pppppppp/8/8/8/8/PPPPPPPP/RNBQKBNR w KQkq - 0 1'
chess.undo()
// -> null

validateFen(fen):

This static function returns a validation object specifying validity or the errors found within the FEN string.

import { validateFen } from 'chess.js'

validateFen('2n1r3/p1k2pp1/B1p3b1/P7/5bP1/2N1B3/1P2KP2/2R5 b - - 4 25')
// -> { ok: true }

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