Код звездного неба, со звездами и падающей кометой

window.requestAnimationFrame = window.requestAnimationFrame || window.mozRequestAnimationFrame ||
  window.webkitRequestAnimationFrame || window.msRequestAnimationFrame;
 
var starDensity = .116;
var speedCoeff = .05;
var width;
var height;
var starCount;
var circleRadius;
var circleCenter;
var first = true;
var giantColor = '180,184,240';
var starColor = '226,225,142';
var cometColor = '226,225,224';
var canva = document.getElementById('universe');
var stars = [];
 
windowResizeHandler();
window.addEventListener('resize', windowResizeHandler, false);
 
createUniverse();
 
function createUniverse() {
  universe = canva.getContext('2d');
 
  for (var i = 0; i < starCount; i++) {
    stars[i] = new Star();
    stars[i].reset();
  }
 
  draw();
}
 
function draw() {
  universe.clearRect(0, 0, width, height);
 
  var starsLength = stars.length;
 
  for (var i = 0; i < starsLength; i++) {
    var star = stars[i];
    star.move();
    star.fadeIn();
    star.fadeOut();
    star.draw();
  }
 
  window.requestAnimationFrame(draw);
}
 
function Star() {
 
  this.reset = function() {
    this.giant = getProbability(3);
    this.comet = this.giant || first ? false : getProbability(10);
    this.x = getRandInterval(0, width - 10);
    this.y = getRandInterval(0, height);
    this.r = getRandInterval(1.1, 2.6);
    this.dx = getRandInterval(speedCoeff, 6 * speedCoeff) + (this.comet + 1 - 1) * speedCoeff * getRandInterval(50, 120) + speedCoeff * 2;
    this.dy = -getRandInterval(speedCoeff, 6 * speedCoeff) - (this.comet + 1 - 1) * speedCoeff * getRandInterval(50, 120);
    this.fadingOut = null;
    this.fadingIn = true;
    this.opacity = 0;
    this.opacityTresh = getRandInterval(.2, 1 - (this.comet + 1 - 1) * .4);
    this.do = getRandInterval(0.0005, 0.002) + (this.comet + 1 - 1) * .001;
  };
 
  this.fadeIn = function() {
    if (this.fadingIn) {
      this.fadingIn = this.opacity > this.opacityTresh ? false : true;
      this.opacity += this.do;
    }
  };
 
  this.fadeOut = function() {
    if (this.fadingOut) {
      this.fadingOut = this.opacity < 0 ? false : true;
      this.opacity -= this.do / 2;
      if (this.x > width || this.y < 0) {
        this.fadingOut = false;
        this.reset();
      }
    }
  };
 
  this.draw = function() {
    universe.beginPath();
 
    if (this.giant) {
      universe.fillStyle = 'rgba(' + giantColor + ',' + this.opacity + ')';
      universe.arc(this.x, this.y, 2, 0, 2 * Math.PI, false);
    } else if (this.comet) {
      universe.fillStyle = 'rgba(' + cometColor + ',' + this.opacity + ')';
      universe.arc(this.x, this.y, 1.5, 0, 2 * Math.PI, false);
 
      //comet tail
      for (var i = 0; i < 30; i++) {
        universe.fillStyle = 'rgba(' + cometColor + ',' + (this.opacity - (this.opacity / 20) * i) + ')';
        universe.rect(this.x - this.dx / 4 * i, this.y - this.dy / 4 * i - 2, 2, 2);
        universe.fill();
      }
    } else {
      universe.fillStyle = 'rgba(' + starColor + ',' + this.opacity + ')';
      universe.rect(this.x, this.y, this.r, this.r);
    }
 
    universe.closePath();
    universe.fill();
  };
 
  this.move = function() {
    this.x += this.dx;
    this.y += this.dy;
    if (this.fadingOut === false) {
      this.reset();
    }
    if (this.x > width - (width / 4) || this.y < 0) {
      this.fadingOut = true;
    }
  };
 
  (function() {
    setTimeout(function() {
      first = false;
    }, 50)
  })()
}
 
function getProbability(percents) {
  return ((Math.floor(Math.random() * 1000) + 1) < percents * 10);
}
 
function getRandInterval(min, max) {
  return (Math.random() * (max - min) + min);
}
 
function windowResizeHandler() {
  width = window.innerWidth;
  height = window.innerHeight;
  starCount = width * starDensity;
  circleRadius = (width > height ? height / 2 : width / 2);
  circleCenter = {
    x: width / 2,
    y: height / 2
  }
 
  canva.setAttribute('width', width);
  canva.setAttribute('height', height);
}