let particle_vel;

function setup() {

  createCanvas(400, 400);

  particle = createVector(width/2, height/2);

  particle_vel = createVector(0,0);

}

​

function draw() {

  background(220);

  circle(particle.x, particle.y, 30);

  particle_vel.y += -1;

  if (particle.y > height) {

    particle_vel.mult(particle_vel.sub(particle_vel * 2));

  } else if (particle.x > width) {

    particle_vel.mult(particle_vel.sub(particle_vel * 2));

  } else if (particle.x < 0) {

    particle_vel.mult(particle_vel.sub(particle_vel * 2));

  } else if (particle.y < 0) {

    particle_vel.mult(particle_vel.sub(particle_vel * 2));

  }

  particle.add(particle_vel);

  print(particle_vel.y);

}