雄辩的Javascript中的Terrarium代码不起作用

这是《 Eloquent Javascript》一书中的代码。它创建了一个称为Terrarium的2D对象，昆虫可以在其中移动。使用function：terrarium.step可以移动昆虫。但是当我运行它时，它说“中心未定义”。center是函数的参数：Terrarium.prototype.listSurroundings。该函数调用另一个函数，该函数使用center方法作为参数。有什么事吗 我在这里附上整个代码，包括玻璃容器的初始化等。

function forEach(array,action) {
    for(var i=0;i<array.length;i++)
    {action(array[i]);}
}

function forEachIn(object, action) {
    for (var property in object) {
        if (Object.prototype.hasOwnProperty.call(object, property))
        action(property, object[property]);
    }
}

function Dictionary(startValues) {
    this.values = startValues || {};
}
Dictionary.prototype.store = function(name, value) {
    this.values[name] = value;
};
Dictionary.prototype.lookup = function(name) {
    return this.values[name];
};

Dictionary.prototype.contains = function(name) {
    return Object.prototype.propertyIsEnumerable.call(this.values, name);
};
Dictionary.prototype.each = function(action) {
    forEachIn(this.values, action);
};

function Point(x, y) {
    this.x = x;
    this.y = y;
}
Point.prototype.add = function(other) {
    return new Point(this.x + other.x, this.y + other.y);
};

function Grid(width, height) {
    this.width = width;
    this.height = height;
    this.cells = new Array(width * height);
}
Grid.prototype.valueAt = function(point) {
    return this.cells[point.y * this.width + point.x];
};
Grid.prototype.setValueAt = function(point, value) {
    this.cells[point.y * this.width + point.x] = value;
};
Grid.prototype.isInside = function(point) {
    return point.x >= 0 && point.y >= 0 &&
    point.x < this.width && point.y < this.height;
};
Grid.prototype.moveValue = function(from, to) {
    this.setValueAt(to, this.valueAt(from));
    this.setValueAt(from, undefined);
};

Grid.prototype.each = function(action) {
    for (var y = 0; y < this.height; y++) {
        for (var x = 0; x < this.width; x++) {
            var point = new Point(x, y);
            action(point, this.valueAt(point));
        }
    }
};

var directions = new Dictionary(
{"n": new Point( 0, -1),
 "ne": new Point( 1, -1),
 "e": new Point( 1, 0),
 "se": new Point( 1, 1),
 "s": new Point( 0, 1),
 "sw": new Point(-1, 1),
 "w": new Point(-1, 0),
 "nw": new Point(-1, -1)});

function StupidBug() {};
StupidBug.prototype.act = function(surroundings) {
    return {type: "move", direction: "s"};
};

var wall = {};

function elementFromCharacter(character) {
    if (character == " ")
        return undefined;
    else if (character == "#")
        return wall;
    else if (character == "o")
        return new StupidBug();
}

function Terrarium(plan) {
    var grid = new Grid(plan[0].length, plan.length);
    for (var y = 0; y < plan.length; y++) {
        var line = plan[y];
        for (var x = 0; x < line.length; x++) {
            grid.setValueAt(new Point(x, y), elementFromCharacter(line.charAt(x)));
        }
    }
    this.grid = grid;
}

wall.character = "#";
StupidBug.prototype.character = "o";
function characterFromElement(element) {
    if (element == undefined)
        return " ";
    else
    return element.character;
}

Terrarium.prototype.toString = function() {
    var characters = [];
    var endOfLine = this.grid.width - 1;
    this.grid.each(function(point, value) {
        characters.push(characterFromElement(value));
        if (point.x == endOfLine)
        characters.push("\n");
    });
    return characters.join("");
};

function bind(func, object) {
    return function(){
        return func.apply(object, arguments);
    };
}

function method(object, name) {
    return function() {
        object[name].apply(object, arguments);
    };
}

Terrarium.prototype.listActingCreatures = function() {
    var found = [];
    this.grid.each(function(point, value) {
        if (value != undefined && value.act)
        found.push({object: value, point: point});
    });
    return found;
};

Terrarium.prototype.listSurroundings = function(center) {
    var result = {};
    var grid = this.grid;
    directions.each(function(name, direction) {
        var place = center.add(direction);
        if (grid.isInside(place))
        result[name] = characterFromElement(grid.valueAt(place));
        else result[name] = "#";
    });
    return result;
};

Terrarium.prototype.processCreature = function(creature, point) {
    var action = creature.act(this.listSurroundings(point));
    if (action.type == "move" && directions.contains(action.direction)) {
        var to = point.add(directions.lookup(action.direction));
        if (this.grid.isInside(to) && this.grid.valueAt(to) == undefined)
            this.grid.moveValue(point, to);
    }
    else {
        throw new Error("Unsupported action: " + action.type);
    }
};

Terrarium.prototype.step = function() {
    forEach(this.listActingCreatures(), bind(this.processCreature, this));
};

var thePlan =
["############################",
 "#      #    #      o      ##",
 "#                          #",
 "#          #####           #",
 "##         #   #    ##     #",
 "###           ##     #     #",
 "#           ###      #     #",
 "#  ####                    #",
 "#  ##        o             #",
 "# o #          o       ### #",
 "#   #                      #",
 "############################"]
var terrarium = new Terrarium(thePlan);
terrarium.step();
alert(terrarium);