let particles = [];

​

function setup() {

  createCanvas(500, 400);

​

  for (let i = 0; i < NUM_OF_PARTICLES; i++) {

    particles.push( new Particle(random(width), random(height)) );

  }

}

​

function draw() {

  background(220);

​

  for (let i = 0; i < particles.length; i++) {

    let p = particles[i];

    p.move();

    p.reappear(); // ***

    p.display();

  }

}

​

//

​

class Particle {

  constructor(x, y) {

    // properties

    this.x = x;

    this.y = y;

    this.xSpd = random(-5, 5);

    this.ySpd = random(-5, 5);

    this.dia = 30;

  }

  // methods

  reappear() {

    if (this.x < 0) {

      this.x = width;

    }

    else if (this.x > width) {

      this.x = 0;

    }

    if (this.y < 0) {

      this.y = height;

    }

    else if (this.y > height) {

      this.y = 0;

    }

  }

  move() {

    this.x += this.xSpd;

    this.y += this.ySpd;

  }

  display() {

    push();

    translate(this.x, this.y);

    circle(0, 0, this.dia);

    pop();

  }

}

​