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Stockfish-js is an emscripten port of the stockfish chess engine. (Emscripten being a c[++] to javascript compiler.) This enables one to run one of the strongest chess engines available without downloads or plugins in a web browser. In Firefox, thanks to its asm.js support, it runs at a respactable 1/3 of the (single-threaded) speed of a native compile on my machine. In Chrome it reaches about half the speed of FF - after a short warm-up time.


Download Stockfish 5 compiled to javascript: stockfish.js

See also

stockfish.js is a fork of this fork which adds some interesting features. Most notably is the ability for the engine to receive commands between iterations, which allows aborting an infitite search (pondering/analysing).


Stockfish-js is designed to run in a web-worker, which can be created like this:

var stockfish = new Worker('stockfish.js');

Input (standard UCI commands) to the engine is posted as a message to the worker:

engine.postMessage('go depth 15');

The output of the engine is again posted as a message, to receive it you need to install an event handler:

engine.onmessage = function(event) {

Since the engine cannot load an opening book from the filesystem, there is a special message "{book: }" to send a book that you have downloaded yourself to the engine:

var bookRequest = new XMLHttpRequest();'GET', 'book.bin', true);
bookRequest.responseType = "arraybuffer";
bookRequest.onload = function(event) {
  if(bookRequest.status == 200)
    engine.postMessage({book: bookRequest.response});


Since there is no support for multi-threading with shared state in browsers, the engine can only run in a single thread. The 'Threads' option is therefore ignored.

The engine cannot receive a command during a search. This means that the 'stop' and 'ponderhit' commands will not work as intended. So pondering make no sense at all, and infinite analysis ('go infinite') only limited sense, as the only way to stop the search is to kill the web-worker and start a new one for the next position to analyze which incurrs quite some overhead and obviously loses the state of the hash.


You need to have the emscripten compiler installed and in your path. Then you can compile Stockfish-js like this:

make build ARCH=js


There is an example implementation of a game against the computer in the example folder. To run this, it needs to be served by a webserver. If you have node.js, you can start a simple webserver in that directory like this:

npm install connect # once to install the connect library
node server.js

Alternatively, there is a online version of this example here:

The example uses the excellent chess.js and chessboard.js libraries.

Original Stockfish Readme:


Stockfish is a free UCI chess engine derived from Glaurung 2.1. It is not a complete chess program and requires some UCI-compatible GUI (e.g. XBoard with PolyGlot, eboard, Arena, Sigma Chess, Shredder, Chess Partner or Fritz) in order to be used comfortably. Read the documentation for your GUI of choice for information about how to use Stockfish with it.

This version of Stockfish supports up to 128 CPUs. The engine defaults to one search thread, so it is therefore recommended to inspect the value of the Threads UCI parameter, and to make sure it equals the number of CPU cores on your computer.


This distribution of Stockfish consists of the following files:

  •, the file you are currently reading.

  • Copying.txt, a text file containing the GNU General Public License.

  • src, a subdirectory containing the full source code, including a Makefile that can be used to compile Stockfish on Unix-like systems. For further information about how to compile Stockfish yourself read section below.

  • polyglot.ini, for using Stockfish with Fabien Letouzey's PolyGlot adapter.

Opening books

This version of Stockfish has support for PolyGlot opening books. For information about how to create such books, consult the PolyGlot documentation. The book file can be selected by setting the Book File UCI parameter.

Compiling it yourself

On Unix-like systems, it should be possible to compile Stockfish directly from the source code with the included Makefile.

Stockfish has support for 32 or 64-bit CPUs, the hardware POPCNT instruction, big-endian machines such as Power PC, and other platforms.

In general it is recommended to run make help to see a list of make targets with corresponding descriptions. When not using the Makefile to compile (for instance with Microsoft MSVC) you need to manually set/unset some switches in the compiler command line; see file types.h for a quick reference.

Terms of use

Stockfish is free, and distributed under the GNU General Public License (GPL). Essentially, this means that you are free to do almost exactly what you want with the program, including distributing it among your friends, making it available for download from your web site, selling it (either by itself or as part of some bigger software package), or using it as the starting point for a software project of your own.

The only real limitation is that whenever you distribute Stockfish in some way, you must always include the full source code, or a pointer to where the source code can be found. If you make any changes to the source code, these changes must also be made available under the GPL.

For full details, read the copy of the GPL found in the file named Copying.txt


UCI chess engine compiled to Javascript







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