snow.js

// Array to store our Snowflake objects
var snowflakes = [];

// Global variables to store our browser's window size
var browserWidth;
var browserHeight;

// Specify the number of snowf`lakes you want visible
var numberOfSnowflakes = 50;

// Flag to reset the position of the snowflakes
var resetPosition = false;

// Handle accessibility
var enableAnimations = false;
var reduceMotionQuery = matchMedia("(prefers-reduced-motion)");

// Handle animation accessibility preferences 
function setAccessibilityState() {
if (reduceMotionQuery.matches) {
  enableAnimations = false;
} else { 
  enableAnimations = true;
}
}
setAccessibilityState();

reduceMotionQuery.addListener(setAccessibilityState);

//
// It all starts here...
//
function setup() {
if (enableAnimations) {
  window.addEventListener("DOMContentLoaded", generateSnowflakes, false);
  window.addEventListener("resize", setResetFlag, false);
}
}
setup();

//
// Constructor for our Snowflake object
//
function Snowflake(element, speed, xPos, yPos) {
// set initial snowflake properties
this.element = element;
this.speed = speed;
this.xPos = xPos;
this.yPos = yPos;
this.scale = 1;

// declare variables used for snowflake's motion
this.counter = 0;
this.sign = Math.random() < 0.5 ? 1 : -1;

// setting an initial opacity and size for our snowflake
this.element.style.opacity = (.1 + Math.random()) / 2;
}

//
// The function responsible for actually moving our snowflake
//
Snowflake.prototype.update = function () {
// using some trigonometry to determine our x and y position
this.counter += this.speed / 5000;
this.xPos += this.sign * this.speed * Math.cos(this.counter) / 40;
this.yPos += Math.sin(this.counter) / 40 + this.speed / 30;
this.scale = .5 + Math.abs(10 * Math.cos(this.counter) / 20);

// setting our snowflake's position
setTransform(Math.round(this.xPos), Math.round(this.yPos), this.scale, this.element);

// if snowflake goes below the browser window, move it back to the top
if (this.yPos > browserHeight) {
  this.yPos = -50;
}
}

//
// A performant way to set your snowflake's position and size
//
function setTransform(xPos, yPos, scale, el) {
el.style.transform = `translate3d(${xPos}px, ${yPos}px, 0) scale(${scale}, ${scale})`;
}

//
// The function responsible for creating the snowflake
//
function generateSnowflakes() {

// get our snowflake element from the DOM and store it
var originalSnowflake = document.querySelector(".snowflake");

// access our snowflake element's parent container
var snowflakeContainer = originalSnowflake.parentNode;
snowflakeContainer.style.display = "block";

// get our browser's size
browserWidth = document.documentElement.clientWidth;
browserHeight = document.documentElement.clientHeight;

// create each individual snowflake
for (var i = 0; i < numberOfSnowflakes; i++) {

  // clone our original snowflake and add it to snowflakeContainer
  var snowflakeClone = originalSnowflake.cloneNode(true);
  snowflakeContainer.appendChild(snowflakeClone);

  // set our snowflake's initial position and related properties
  var initialXPos = getPosition(50, browserWidth);
  var initialYPos = getPosition(50, browserHeight);
  var speed = 5 + Math.random() * 40;

  // create our Snowflake object
  var snowflakeObject = new Snowflake(snowflakeClone,
    speed,
    initialXPos,
    initialYPos);
  snowflakes.push(snowflakeObject);
}

// remove the original snowflake because we no longer need it visible
snowflakeContainer.removeChild(originalSnowflake);

moveSnowflakes();
}

//
// Responsible for moving each snowflake by calling its update function
//
function moveSnowflakes() {

if (enableAnimations) {
  for (var i = 0; i < snowflakes.length; i++) {
    var snowflake = snowflakes[i];
    snowflake.update();
  }      
}

// Reset the position of all the snowflakes to a new value
if (resetPosition) {
  browserWidth = document.documentElement.clientWidth;
  browserHeight = document.documentElement.clientHeight;

  for (var i = 0; i < snowflakes.length; i++) {
    var snowflake = snowflakes[i];

    snowflake.xPos = getPosition(50, browserWidth);
    snowflake.yPos = getPosition(50, browserHeight);
  }

  resetPosition = false;
}

requestAnimationFrame(moveSnowflakes);
}

//
// This function returns a number between (maximum - offset) and (maximum + offset)
//
function getPosition(offset, size) {
return Math.round(-1 * offset + Math.random() * (size + 2 * offset));
}

//
// Trigger a reset of all the snowflakes' positions
//
function setResetFlag(e) {
resetPosition = true;
}