Simple Circle Collision

sketch.js

var balls = [];

function setup() {
  createCanvas(windowWidth,windowHeight);
  x = width/2;
  y = height/2;
}

function draw() {
  background(255,255,255); 
  for (var i = 0; i < balls.length; i++) {
    balls[i].display();
  }
//check collision
  for (var i = 0; i < balls.length; i++) {
    for (var j = i + 1; j < balls.length; j++) {
      balls[i].hitCheck(balls[j]);
    }
  }
}

function mousePressed() {
  balls.push(new Ball());
}
var Ball = function() {
  this.x = mouseX;
  this.y = mouseY;
  this.speedX = random(-1,1);
  this.diameter = random(20,50);
}

Ball.prototype.display = function() {
  ellipse(this.x, this.y, this.diameter, this.diameter);
  this.x += this.speedX;
}

Ball.prototype.hitCheck = function(otherBall) {
  if (dist(this.x, this.y, otherBall.x, otherBall.y) < this.diameter) {
    this.speedX *= -1;
    otherBall.speedX *= -1;
  }
  
}