chess960.js

var startStr = "rnbqkbnr/pppppppp/8/8/8/8/PPPPPPPP/RNBQKBNR w KQkq - 0 1";

var board = [ 'rnbqkbnr','pppppppp','8','8','8','8','PPPPPPPP','RNBQKBNR'];
var meta = ' w KQkq - 0 1';

function generateChess960() {
    var C960Array = new Array(8);
    var str = "01234567";
    var Bwx = (Math.floor(Math.random() * 4) * 2) + 1;
    C960Array[Bwx] = "B";
    var Bbx = Math.floor(Math.random() * 4) * 2;
    C960Array[Bbx] = "B";
    for (var i = str.length - 1; i >= 0; i--) {
        if ((str.charAt(i) == Bbx) || (str.charAt(i) == Bwx))
            str = str.substr(0, i) + str.substr(i + 1);
    }
    var Qx = Math.floor(Math.random() * 6);
    C960Array[str.charAt(Qx)] = "Q";
    str = str.substr(0, Qx) + str.substr(Qx + 1);
    var Nx = Math.floor(Math.random() * 5);
    C960Array[str.charAt(Nx)] = "N";
    str = str.substr(0, Nx) + str.substr(Nx + 1);
    Nx = Math.floor(Math.random() * 4);
    C960Array[str.charAt(Nx)] = "N";
    str = str.substr(0, Nx) + str.substr(Nx + 1);
    C960Array[str.charAt(0)] = "R";
    str = str.substr(1);
    C960Array[str.charAt(0)] = "K";
    str = str.substr(1);
    C960Array[str] = "R";
    return C960Array.join("");
}

function Chess960Number(str) {
    var Bb, Bw;
    var tempStr = str.replace(/Q|B/g, "");
    var N1 = tempStr.indexOf("N");
    var N2 = tempStr.lastIndexOf("N");
    tempStr = str.replace(/B/g, "");
    var Q = tempStr.indexOf("Q");
    for (var i = 0; i < 4; i++) {
        if (str.charAt(i * 2) == "B")
            Bb = i;
        if (str.charAt((i * 2) + 1) == "B")
            Bw = i;
    }
    return (96 * (5 - (((3 - N1) * (4 - N1)) / 2) + N2)) + (16 * Q) + (4 * Bb) + Bw;
}

function generate960FEN() {
    var x = generateChess960();
    board[0] = x.toLowerCase();
    board[7] = x.toUpperCase();
    return board.join("/") + meta;
}

function generateFEN() {
    return startStr;
}

console.log(generateFEN());
console.log(generate960FEN());