为什么一模一样的代码，我自己敲一遍，就运行不出来相同的效果？

var WINDOW_WIDTH=1024;

var WINDOW_HEIGHT=768;

var WINDOW_TOP=60;

var WINDOW_LEFT=30;

var RADIUS=10;

window.onload = function () {

    var canvas=document.getElementsById('canvas');    //调用canvas

    var context=canvas.getContext("2d");   //调用绘图的上下文环境

    canvas.width=WINDOW_WIDTH;

    canvas.height=WINDOW_HEIGHT;  //调用全局变量

    render(context)   //绘图函数

};

function render(cxt) {

    var hours=12;

    var minutes=34;

    var seconds=56;   //定义时间

    renderDigit(WINDOW_LEFT,WINDOW_TOP,parseInt(hours/10),cxt);  //将数拆开，一个数一个数的绘制

    renderDigit(WINDOW_LEFT+15*(RADIUS+1),WINDOW_TOP,parseInt(hours%10),cxt);

    renderDigit(WINDOW_LEFT+30*(RADIUS+1),WINDOW_TOP,10,cxt);

    renderDigit(WINDOW_LEFT+39*(RADIUS+1),WINDOW_TOP,parseInt(minutes/10),cxt);

    renderDigit(WINDOW_LEFT+54*(RADIUS+1),WINDOW_TOP,parseInt(minutes%10),cxt);

    renderDigit(WINDOW_LEFT+69*(RADIUS+1),WINDOW_TOP,10,cxt);

    renderDigit(WINDOW_LEFT+78*(RADIUS+1),WINDOW_TOP,parseInt(seconds/10),cxt);

    renderDigit(WINDOW_LEFT+93*(RADIUS+1),WINDOW_TOP,parseInt(seconds%10),cxt);

}

function renderDigit(x,y,num,cxt){

    cxt.fillStyle="rag(0,102,153)";

    for(var i=0;i<digit[num].length;i++)   //i代表行，j代表列，num代表数，.length代表方法

        for(var j=0;j<digit[num][i].length;j++)     //1是矩阵里的，代表绘制小球

        {

            if(digit[num][i][j] == 1) {

                cxt.beginPath();  //绘图的一个区间

                cxt.arc(x + j * 2 * (RADIUS + 1) + (RADIUS + 1), y + i * 2 * (RADIUS + 1) + (RADIUS + 1), RADIUS, 0, 2 * Math.PI);

                cxt.closePath();

                cxt.fill()

            }

        }

}

这有什么错误？为什么运行不了？