Be aware that processing.js cannot save a file to your harddisk, because of the javascript security model. Instead, pjs will place the image in a new window for you to manually save to disk. The following code is a demonstration of this:

1. void setup()
2. {
3.   size(200,200);
4.   noLoop();
5. }
6. 
   
7. void draw()
8. {
9.   background(0);
10.   for(int i=0; i<1000; i++)
11.   {
12.     stroke(random(100,255),random(100,255),random(100,255),15);
13.     line(0,0,width,random(height));
14.     line(0,0,random(width),height);
15.   }
16.   save("mypic.png");
17. }