function draw() {  
      var canvas = document.getElementById('canvas');  
      if (canvas.getContext) {  
        var context = canvas.getContext('2d');  
      
        context.fillRect(20,20,100,100);  
        context.clearRect(40,40,60,60);  
        context.strokeRect(45,45,50,50);  
      }  
    }