Javascript 未定义函数错误

我正在用JavaScript构建Conway's Game of Life。上一个问题已修复。........................................................................现在它说：无法读取在"for（var i =0; i<this.checkneighbors.length;>

//object constructor
function cell(){
    this.alive = Math.random() > 0.7;   
    this.neighbours = 0;  //number of live neighbours
    this.checkneighbours = [[-1,-1],[-1,0],[0,-1],[-1,1],[1,-1],[1,0],[0,1],[1,1]];
}

function GoL(size){
    this.size = size;
    this.grid = this.makeGrid(size);
};      
GoL.prototype.makeGrid = function(size){
    var grid = [];
    for(var i=0; i<size; i++){
        var row=[];
        for(var j =0; j<size; j++){
            row.push(new cell());   
        }
        grid.push(row);
    }
    return grid;
};  
GoL.prototype.drawGrid = function(){
    for(var i=0;i<this.size;i++){
        var row =this.grid[i];
        var rowCell="";
        for(var j=0;j<this.size;j++){
            var cell = row[j];
            if(cell.alive){
                rowCell += "X|";
            }else{
                rowCell += " |";
            }               
        }
        console.log(rowCell);
    }       
};  
GoL.prototype.underpopulation = function(ro,col){
    var cell = this.grid[ro][col];
    if(cell.neighbours <2){
        return true;
    }else{
        return false;   
    }
};  
GoL.prototype.overpopulation = function(ro,col){
    var cell = this.grid[ro][col];
    if(cell.neighbours >3){
        return true;
    }else{
        return false;   
    }
};  
GoL.prototype.backtolife = function(ro,col){
    var cell = this.grid[ro][col];
    if(cell.neighbours ===3 && !cell.alive){
        return true;
    }else{
        return false;   
    }   
};
GoL.prototype.update = function(ro,col){    
    var cell = this.grid[ro][col];
    cell.num_of_neighbours = 0;
    for(var i =0; i<this.checkneighbours.length; i++){
        var checkneighbour = this.checkneighbours[i];
        var neighbour1 = direction[0];
        var neighbour2 = direction[1];
        if(neighbour1>=0 && neighbour1 < this.size && neighbour2 >=0 && neighbour2 < this.size){
            var currentneighbour = this.grid[ro + neighbour1][col+neighbour2];
            if(currentneighbour.alive){
                cell.num_of_neighbours++;
            }
        }
    }
};
GoL.prototype.updateAll = function(){
    for(var i=0; i<this.size;i++){
        for(var j=0; j<this.size;j++){
            this.update(i,j);
        }
    }   
}
GoL.prototype.cellstatus = function(ro,col){
    var cell = this.grid[ro][col];
    if(this.underpopulation(ro,col) || this.overpopulation(ro,col)){
        cell.alive = false;
    }else if(this.backtolife(ro,col)){
        cell.alive = true;
    }
};
GoL.prototype.allcellstatus = function(ro,col){
    for(var i=0; i<this.size;i++){
        for(var j=0; j<this.size;j++){
            this.cellstatus(i,j);
        }
    }   
};

var gameoflife = new GoL(40);   
var interval = setInterval(function(){
    GoL.drawGrid();
    GoL.updateAll();
    GoL.allcellstatus();
},200);