Chess engine View source on BitBucket

CeruleanJS is an XBoard chess engine for NodeJS and the browser. It is currently ranked on CCRL 40/4 with an ELO of approximately 1300. It plays on FICS under the registered computer account CeruleanJS. Click here to view CeruleanJS's FICS games.

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 /'___\ /'__`\/\`'__\/\ \/\ \ \ \ \   /'__`\ /'__`\  /' _ `\
/\ \__//\  __/\ \ \/ \ \ \_\ \ \_\ \_/\  __//\ \L\.\_/\ \/\ \
\ \____\ \____\\ \_\  \ \____/ /\____\ \____\ \__/.\_\ \_\ \_\
 \/____/\/____/ \/_/   \/___/  \/____/\/____/\/__/\/_/\/_/\/_/

Build Status


CeruleanJS is an XBoard chess engine for NodeJS, written by Joey Robert.

The goals of CeruleanJS are threefold:

  • Correctness
  • Performance
  • Ease of programming


  • Node >= 4.0


To install the binary into your $PATH, install the ceruleanjs package globally:

npm install -g ceruleanjs

Run the engine with the command:


If you’re installing from sources, check out this git repository and run:

npm install

To run:

npm start

To run mocha tests:

npm test

To generate a Windows binary executable using nexe, run:

npm run build-windows

This requires Microsoft Visual Studio 2015 to be installed. It will generate ceruleanjs.exe and book.bok in dist.

To generate a web build, run:

npm run build-web

This will generate a babelify’d ES5 version of CeruleanJS that is compatible with most modern day web browsers that support Web Workers. This build is used in the CeruleanJS Player.


CeruleanJS supports the XBoard/winboard/CECP format for communicating with interfaces or other chess engines. It’s also usable directly through STDIN/OUT. Here’s the list of commands:


display                     Draws the board
perft [INT]                 Perfts the current board to specified depth
perfthash [INT]             Sets perft hashtable exponent (size 2^exponent)
divide [INT]                Divides the current board to specified depth
moves                       Lists valid moves for this position
e2e4                        Moves from the current position and thinks
go                          Forces the engine to think
undo                        Subtracts the previous move
new                         Sets up the default board position
setboard [FEN]              Sets the board using Forsyth-Edwards Notation
evaluate                    Performs a static evaluation of the board
result                      Displays game result (checkmate or stalemate)
book [on|off]               Toggles whether engine uses opening book
white                       Sets the active colour to WHITE
black                       Sets the active colour to BLACK
time [INT]                  Sets engine's time (in centiseconds)
otim [INT]                  Sets opponent's time (in centiseconds)
sd [INT]                    Sets maximum depth
st [INT]                    Sets maximum time
level [MPT] [BASE] [INC]    Sets Winboard level timing
sts                         Run Strategic Test Suite (1s per move)
version                     Outputs the version number
exit                        Exits the menu
quit                        See exit
help                        Gets you this magical menu

Interface GIF

CeruleanJS Usage GIF


For a more technical breakdown of how CeruleanJS is implemented, see docs/TECHNICAL.md


CeruleanJS is licensed under the GNU GPL v3.


  • v0.1.1 (Blizzard) - 2016-01-20

    • Fixed Polyglot castling bug
  • v0.1.0 (Blizzard) - 2016-01-20

    • Vastly improved move generation performance (10x)
    • Polyglot opening book (removes Mersenne Twister dependency)
      • Changed piece representation to Polyglot format
      • Implemented Polyglot Zobrist key support
      • Changed turn value (BLACK = 0, WHITE = 1)
    • Improved time management (supporting winboard level)
    • Implemented Strategic Test Suite (STS)
    • Support for Standard Algebraic Notation through moves command
    • Implemented Static Exchange Evaluation (SEE) – not used at the moment
    • Implemented MVV/LVA move ordering
    • Improved evaluation function
    • Denser move structure
    • Added ANSI colors function (removes colors dependency)
    • Switched to Node’s readline (removes stdio dependency)
    • Web support through CeruleanJS Player
    • Fixed Windows EXE generation bug
    • Improved unit test coverage
    • Changed LICENSE from GPLv2 to GPLv3
    • Rating roughly ~1300 ELO
  • v0.0.1 (Azure) - 2015-12-13

    • 15x12 board representation
    • Black and White piece lists
    • Move generation passing 100% perft test suite
    • FEN board getter and setter
    • 53-bit zobrist hash
    • Hash table
    • PVS Search with quiescence
    • Iterative deepening with basic move ordering
    • Simplified evaluation function (material + piece square tables)
    • Windows EXE generation
    • XBoard support

For technical details and to play the CeruleanJS online, please visit the CeruleanJS homepage.


Play CeruleanJS chess online