javascript - my canvas is null the browser is blank -


i write little web game html5 plus javascript. create 2 file ,one htmml5 file javascript file.here come problem canvas null. here html code

<!doctype html> <html> <head>     <title>frogger</title> <script type="text/javascript" src="gamecontrol.js"></script> </head> <body> <canvas id="painting" width="600" height="600" tabindex="0" style="border: solid blue 1px">  </canvas>​  </body> </html>

my javascript code

var canvas = document.getelementbyid("painting");     canvas.addeventlistener("keydown", function(e) {           //listen key event         dokeydown(e);     });      var context2d = canvas.getcontext("2d");      var lifecount = 3;     // frogger has 3 life values     var score = 0;         // when frogger accross line 10 points      var pic = new image();     var pic1=new image();     var picpipe1 = new image();      // pipe image     var picpipe2 = new image();     var spider1 = new image();     var spider2 = new image();     var gresp1 = new image();     var gresp2 = new image();     var rocket = new image();     var car = new image();     var wheel = new image();     var car1 = new image();     var car2 = new image();      //the coordinates     var x=0,y=110;     var spider1x= 0,spider1y=140;     var spider2x= 0,spider2y=230;     var x1=300,y1=450;     // frogger's ordinary coordinate     var pipe1x=0,pipe1y=170;     var pipe2x= 0,pipe2y=200;     var rocketx = 0,rockety=300;     var carx = 0, cary =330;     var wheelx= 0,wheely=360;     var car1x= 0, car1y=390;     var car2x= 0, car2y=420;      //load image     pic.src = "pipe.jpg";     pic1.src = "qingwa.jpg";     picpipe1.src = "pipe.jpg";     picpipe2.src = "pipe.jpg";     spider1.src="spider.jpg";     spider2.src="spider.jpg";     gresp1.src = "gresp.jpg";     gresp2.src = "gresp.jpg";      rocket.src = "rocket.jpg";     car.src = "car.jpg";     wheel.src = "wheel.jpg";     car1.src = "car1.jpg";     car2.src = "car2.jpg";      function drawline()     {     context2d.moveto(0,100);     context2d.lineto(35,100);     context2d.lineto(35,60);     context2d.lineto(85,60);     context2d.lineto(85,100);     context2d.lineto(155,100);     context2d.lineto(155,60);     context2d.lineto(205,60);     context2d.lineto(205,100);     context2d.lineto(275,100);     context2d.lineto(275,60);     context2d.lineto(325,60);     context2d.lineto(325,100);     context2d.lineto(395,100);     context2d.lineto(395,60);     context2d.lineto(445,60);     context2d.lineto(445,100);     context2d.lineto(515,100);     context2d.lineto(515,60);     context2d.lineto(565,60);     context2d.lineto(565,100);     context2d.lineto(600,100);       context2d.stroke();     }     function dokeydown(e)     {         switch (e.keycode)         {         // key             case 38:             y1 = y1 - 45;             break;         //the down key             case 40:             y1 = y1 + 45;             break;          //the left key             case 37:             x1 = x1 - 10;             break;          //the right key             case 39:             x1 = x1 + 10;             break;          }     }      function drawfrogger()     {          context2d.drawimage(pic1,x1,y1);     }     function draw()     {          context2d.clearrect(0,0,600,600);         context2d.save();          context2d.drawimage(gresp1,0,250);         context2d.drawimage(gresp2,0,450);         context2d.drawimage(pic,x,y);         context2d.drawimage(picpipe1,pipe1x,pipe1y);         context2d.drawimage(picpipe2,pipe2x,pipe2y);         context2d.drawimage(spider1,spider1x,spider1y);         context2d.drawimage(spider2,spider2x,spider2y);         context2d.drawimage(rocket,rocketx,rockety);         context2d.drawimage(car,carx,cary);         context2d.drawimage(wheel,wheelx,wheely);         context2d.drawimage(car1,car1x,car1y);         context2d.drawimage(car2,car2x,car2y);         //some text score line         context2d.font = "40pt calibri";         context2d.fillstyle = "blue";         context2d.filltext('score:',0,550);          context2d.font = "40pt calibri";         context2d.fillstyle = "red";         context2d.filltext(score,180,550);          x = x + 100;         pipe1x = pipe1x + 60;         pipe2x = pipe2x + 80;         spider1x = spider1x + 70;         spider2x = spider2x + 55;         rocketx = rocketx + 35;         carx = carx + 25;         wheelx = wheelx + 55;         car1x =car1x + 45;         car2x = car2x + 38;          drawfrogger();         drawline();         context2d.restore(); //绘制结束以后，恢复画笔状态         if(x>600)             x = 0;         if(pipe1x > 600)             pipe1x = 0;          if(pipe2x > 600)         pipe2x = 0;          if(spider1x > 600)         spider1x = 0;          if(spider2x > 600)         spider2x = 0;          if(rocketx > 600)         rocketx = 0;          if(carx > 600)         carx = 0;          if(wheelx > 600)         wheelx = 0;          if(car1x > 600)         car1x = 0;          if(car2x > 600)         car2x = 0;     }      //collision test      function collision()     {         switch(y1)         {             case 405:                 score = 10;                 break;              case 360:                 score = 20;                 break;             case 315:                 score = 30;                 break;             case 270:                 score = 40;                 break;             case 225:                 score = 50;                 break;           }      }       function run()     {         draw();         collision()      }     setinterval(run, 1000);

the reason error attempting access canvas element before dom ready.

making following modification js should sovle it.

window.onload = function() {     var canvas = document.getelementbyid("painting");     ...     ...      car2.src = "car2.jpg"; };  function drawline() {     ... }  ... rest of functions

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