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Create fireworks using javascript

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<!DOCTYPE HTML>
<html>
<head>
<meta charset="utf-8">
<title>Fireworks!</title>
<script>
let SCREEN_WIDTH = window.innerWidth,
SCREEN_HEIGHT = window.innerHeight,
mousePos = {
x: 400,
y: 300
},
// create canvas
canvas = document.createElement('canvas'),
context = canvas.getContext('2d'),
particles = [],
rockets = [],
MAX_PARTICLES = 400,
colorCode = 0;
// update mouse position
addEventListener("mousemove", function(e) {
e.preventDefault();
mousePos = {
x: e.clientX,
y: e.clientY
};
});
// launch more rockets!!!
addEventListener("mousedown", function(e) {
for (let i = 0; i < 5; i++) {
launchFrom(Math.random() * SCREEN_WIDTH * 2 / 3 + SCREEN_WIDTH / 6);
}
});
function launch() {
launchFrom(mousePos.x);
}
function launchFrom(x) {
if (rockets.length < 10) {
let rocket = new Rocket(x);
rocket.explosionColor = Math.floor(Math.random() * 360 / 10) * 10;
rocket.vel.y = Math.random() * -3 - 4;
rocket.vel.x = Math.random() * 6 - 3;
rocket.size = 8;
rocket.shrink = 0.999;
rocket.gravity = 0.01;
rockets.push(rocket);
}
}
function loop() {
// update screen size
if (SCREEN_WIDTH != window.innerWidth) {
canvas.width = SCREEN_WIDTH = window.innerWidth;
}
if (SCREEN_HEIGHT != window.innerHeight) {
canvas.height = SCREEN_HEIGHT = window.innerHeight;
}
// clear canvas
context.fillStyle = "rgba(0, 0, 0, 0.05)";
context.fillRect(0, 0, SCREEN_WIDTH, SCREEN_HEIGHT);
let existingRockets = [];
for (let i = 0; i < rockets.length; i++) {
// update and render
rockets[i].update();
rockets[i].render(context);
// calculate distance with Pythagoras
let distance = Math.sqrt(Math.pow(mousePos.x - rockets[i].pos.x, 2) + Math.pow(mousePos.y - rockets[i].pos.y, 2));
// random chance of 1% if rockets is above the middle
let randomChance = rockets[i].pos.y < (SCREEN_HEIGHT * 2 / 3) ? (Math.random() * 100 <= 1) : false;
/* Explosion rules
- 80% of screen
- going down
- close to the mouse
- 1% chance of random explosion
*/
if (rockets[i].pos.y < SCREEN_HEIGHT / 5 || rockets[i].vel.y >= 0 || distance < 50 || randomChance) {
rockets[i].explode();
} else {
existingRockets.push(rockets[i]);
}
}
rockets = existingRockets;

let existingParticles = [];

for (let i = 0; i < particles.length; i++) {
particles[i].update();
// render and save particles that can be rendered
if (particles[i].exists()) {
particles[i].render(context);
existingParticles.push(particles[i]);
}
}
// update array with existing particles - old particles should be garbage collected
particles = existingParticles;
while (particles.length > MAX_PARTICLES) {
particles.shift();
}
}
function Particle(pos) {
this.pos = {
x: pos ? pos.x : 0,
y: pos ? pos.y : 0
};
this.vel = {
x: 0,
y: 0
};
this.shrink = .97;
this.size = 2;

this.resistance = 1;
this.gravity = 0;

this.flick = false;

this.alpha = 1;
this.fade = 0;
this.color = 0;
}
Particle.prototype.update = function() {
// apply resistance
this.vel.x *= this.resistance;
this.vel.y *= this.resistance;

// gravity down
this.vel.y += this.gravity;

// update position based on speed
this.pos.x += this.vel.x;
this.pos.y += this.vel.y;

// shrink
this.size *= this.shrink;

// fade out
this.alpha -= this.fade;
};
Particle.prototype.render = function(c) {
if (!this.exists()) {
return;
}
c.save();
c.globalCompositeOperation = 'lighter';

let x = this.pos.x,
y = this.pos.y,
r = this.size / 2;

let gradient = c.createRadialGradient(x, y, 0.1, x, y, r);
gradient.addColorStop(0.1, "rgba(255,255,255," + this.alpha + ")");
gradient.addColorStop(0.8, "hsla(" + this.color + ", 100%, 50%, " + this.alpha + ")");
gradient.addColorStop(1, "hsla(" + this.color + ", 100%, 50%, 0.1)");

c.fillStyle = gradient;

c.beginPath();
c.arc(this.pos.x, this.pos.y, this.flick ? Math.random() * this.size : this.size, 0, Math.PI * 2, true);
c.closePath();
c.fill();

c.restore();
};

Particle.prototype.exists = function() {
return this.alpha >= 0.1 && this.size >= 1;
};

function Rocket(x) {
Particle.apply(this, [{
x: x,
y: SCREEN_HEIGHT}]);
this.explosionColor = 0;
}

Rocket.prototype = new Particle();
Rocket.prototype.constructor = Rocket;

Rocket.prototype.explode = function() {
let count = Math.random() * 10 + 80;

for (let i = 0; i < count; i++) {
let particle = new Particle(this.pos);
let angle = Math.random() * Math.PI * 2;

// emulate 3D effect by using cosine and put more particles in the middle
let speed = Math.cos(Math.random() * Math.PI / 2) * 15;

particle.vel.x = Math.cos(angle) * speed;
particle.vel.y = Math.sin(angle) * speed;

particle.size = 10;

particle.gravity = 0.2;
particle.resistance = 0.92;
particle.shrink = Math.random() * 0.05 + 0.93;

particle.flick = true;
particle.color = this.explosionColor;

particles.push(particle);
}
};
Rocket.prototype.render = function(c) {
if (!this.exists()) {
return;
}
c.save();
c.globalCompositeOperation = 'lighter';
let x = this.pos.x,
y = this.pos.y,
r = this.size / 2;

let gradient = c.createRadialGradient(x, y, 0.1, x, y, r);
gradient.addColorStop(0.1, "rgba(255, 255, 255 ," + this.alpha + ")");
gradient.addColorStop(1, "rgba(0, 0, 0, " + this.alpha + ")");

c.fillStyle = gradient;

c.beginPath();
c.arc(this.pos.x, this.pos.y, this.flick ? Math.random() * this.size / 2 + this.size / 2 : this.size, 0, Math.PI * 2, true);
c.closePath();
c.fill();

c.restore();
};
</script>
<style>
body {
background-color: #000000;
margin: 0px;
overflow: hidden;
}
</style>
</head>
<body>
<script>
document.body.appendChild(canvas);
canvas.width = SCREEN_WIDTH;
canvas.height = SCREEN_HEIGHT;
setInterval(launch, 800);
setInterval(loop, 1000 / 50);
</script>
</body>
</html>