Prim's Algorithm

var UP = 1, DOWN = 2, RIGHT = 4, LEFT = 8;
var BOXCHARS =[" ", "\u2551", "\u2551", "\u2551", "\u2550", "\u255A", "\u2554", "\u2560",
               "\u2550", "\u255D", "\u2557", "\u2563", "\u2550", "\u2569", "\u2566", "\u256C"];

function Wall(x1, y1, x2, y2) {
    this.x1 = x1;
    this.y1 = y1;
    this.x2 = x2;
    this.y2 = y2;
}
Wall.prototype.is = function (other) {
    return (this.x1 == other.x1 && this.y1 == other.y1 && this.x2 == other.x2 && this.y2 == other.y2)
        || (this.x1 == other.x2 && this.y1 == other.y2 && this.x2 == other.x1 && this.y2 == other.y1);
};
Wall.has = function (list, wall) {
    for (var i = 0; i < list.length; i++) {
        if (wall.is(list[i]))
            return true;
    }
    return false;
};

function maze(width, height, startx, starty, endx, endy) {
    var cells = [];
    for (var i = 0; i < width; i++) {
        cells[i] = [];
        for (var j = 0; j < height; j++) {
            cells[i][j] = 0;
        }
    }
    var walls = [];
    if (startx > 0) {
        cells[startx][starty] |= LEFT;
        walls.push(new Wall(startx, starty, startx - 1, starty));
    }
    if (starty > 0) {
        cells[startx][starty] |= UP;
        walls.push(new Wall(startx, starty, startx, starty - 1));
    }
    if (startx < width - 1) {
        cells[startx][starty] |= RIGHT;
        walls.push(new Wall(startx, starty, startx + 1, starty));
    }
    if (starty < height - 1) {
        cells[startx][starty] |= DOWN;
        walls.push(new Wall(startx, starty, startx, starty + 1));
    }
    var result = [];
    while (walls.length > 0) {
        var index = Math.floor(Math.random() * walls.length);
        var wall = walls.splice(index, 1)[0];
        if (cells[wall.x2][wall.y2] == 0) {
            if (wall.x2 - wall.x1 == 1) { // moved right
                cells[wall.x1][wall.y1] |= RIGHT;
                cells[wall.x2][wall.y2] |= LEFT;
            } else if (wall.x2 > 0)
                walls.push(new Wall(wall.x2, wall.y2, wall.x2 - 1, wall.y2));
            if (wall.x2 - wall.x1 == -1) { // moved left
                cells[wall.x1][wall.y1] |= LEFT;
                cells[wall.x2][wall.y2] |= RIGHT;
            } else if (wall.x2 < width - 1)
                walls.push(new Wall(wall.x2, wall.y2, wall.x2 + 1, wall.y2));
            if (wall.y2 - wall.y1 == 1) { // moved down
                cells[wall.x1][wall.y1] |= DOWN;
                cells[wall.x2][wall.y2] |= UP;
            } else if (wall.y2 > 0)
                walls.push(new Wall(wall.x2, wall.y2, wall.x2, wall.y2 - 1));
            if (wall.y2 - wall.y1 == -1) { // moved up
                cells[wall.x1][wall.y1] |= UP;
                cells[wall.x2][wall.y2] |= DOWN;
            } else if (wall.y2 < height - 1)
                walls.push(new Wall(wall.x2, wall.y2, wall.x2, wall.y2 + 1));
        } else
            result.push(wall);
    }
    var gwidth = 2 * width + 1, gheight = 2 * height + 1;
    var grid = [];
    for (var i = 0; i < gwidth; i++) {
        grid[i] = [];
        for (var j = 0; j < gheight; j++) {
            grid[i][j] = BOXCHARS[0];
        }
    }
    for (var x = 0; x < gwidth; x++) {
        for (var y = 0; y < gheight; y++) {
            var dirs = 0;
            if ((x == 0 || x == gwidth - 1) && y > 0)
                dirs |= UP
            if ((x == 0 || x == gwidth - 1) && y < gheight - 1)
                dirs |= DOWN
            if ((y == 0 || y == gheight - 1) && x > 0)
                dirs |= LEFT
            if ((y == 0 || y == gheight - 1) && x < gwidth - 1)
                dirs |= RIGHT
            // Magic. Don't touch.
            if (x % 2 + y % 2 == 0) {
                if (Wall.has(result, new Wall(x / 2 - 1, y / 2 - 1, x / 2, y / 2 - 1)))
                    dirs |= UP
                if (Wall.has(result, new Wall(x / 2 - 1, y / 2, x / 2, y / 2)))
                    dirs |= DOWN
                if (Wall.has(result, new Wall(x / 2 - 1, y / 2 - 1, x / 2 - 1, y / 2)))
                    dirs |= LEFT
                if (Wall.has(result, new Wall(x / 2, y / 2 - 1, x / 2, y / 2)))
                    dirs |= RIGHT
            }
            grid[x][y] = BOXCHARS[dirs];
        }
    }
    for (var x = 0; x < width; x++) {
        for (var y = 0; y < height; y++) {
            // Magic. Don't touch.
            if (Wall.has(result, new Wall(x, y, x + 1, y)))
                grid[2 * x + 2][2 * y + 1] = BOXCHARS[UP | DOWN];
            if (Wall.has(result, new Wall(x, y, x, y + 1)))
                grid[2 * x + 1][2 * y + 2] = BOXCHARS[LEFT | RIGHT];
        }
    }
    return grid;
}