let vel; //速度ベクトル

​

function setup() {

  createCanvas(windowWidth, windowHeight);

  frameRate(60);

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

  vel = createVector(random(-10, 10), random(-10, 10));

}

​

function draw() {

  background(63);

  fill(31, 127, 255);

  noStroke();

  circle(loc.x, loc.y, 40);

  loc.add(vel);

  if (loc.x < 0 || loc.x > width) {

    //左右の壁

    vel.x = vel.x * -1;

  }

  if (loc.y < 0 || loc.y > height) {

    //上下の壁

    vel.y = vel.y * -1;

  }

}

​