draw_dots_in_update_function.js

// Clear the canvas and draw background:
draw_background();

// there is one number in dots_x for each dot, so dots_x.length is equal to the number of dots.
// For every dot:
for (i = 0; i < dots_x.length; i++)
{
    // Move dot down canvas a little:
    dots_y[i] += dot_pixels_per_frame;
    canvas.beginPath(); // Start drawing
    canvas.arc(dots_x[i], dots_y[i], dot_radius, 0, 2*Math.PI); // Draw 360 degree arc (full circle)
    canvas.fillStyle = dots_color[i]; // Set color
    canvas.fill(); // Fill in circle
    canvas.closePath(); // End drawing
}