let world;

//list of particles

let particles = [];

//an object to store information about the uneven surface

let surface;

​

function setup() {

  createCanvas(560, 390);

​

  //initialize box2d physics and create the world

  world = createWorld();

​

  //create the surface

  surface = new Surface();

}

​

function draw() {

  background(220);

​

  //we must always step through time

  let timeStep = 1.0 / 30;

  //second and third arguments are velocity and position iterations

  world.Step(timeStep, 10, 10);

​

  //particles fall from the top every so often

  if (random(1) < 0.5) {

    let sz = random(4, 8);

    particles.push(new Particle(width / 2, 10, sz));

  }

​

  //draw the surface

  surface.display();

​

  //display all the particles

  for (let i = particles.length - 1; i >= 0; i--) {

    particles[i].display();

    if (particles[i].done()) {

      particles.splice(i, 1);

    }

  }

}