基于JavaScript实现新年贺卡特效

html, body {
            margin: 0;
            padding: 0;
            overflow: hidden;
            background: #000;
            font-family: Montserrat, sans-serif;
            background-image: url(img/pexels-photo-219692.jpeg);
            background-size: cover;
            background-position: center;
        }

html, body {
            margin: 0;
            padding: 0;
            overflow: hidden;
            background: #000;
            font-family: Montserrat, sans-serif;
            background-image: url(img/pexels-photo-219692.jpeg);
            background-size: cover;
            background-position: center;
        }
 
        canvas {
            mix-blend-mode: lighten;
            cursor: pointer;
        }
 
        #hero {
            display: inline;
            position: absolute;
            top: 50%;
            left: 50%;
            transform: translate(-50%, -50%);
            mix-blend-mode: color-dodge;
        }
 
        #year {
            font-size: 30vw;
            color: #d0d0d0;
            font-weight: bold;
        }
 
        #timeleft {
            color: #fbfbfb;
            font-size: 2.5vw;
            text-align: center;
            font-family: Lora, serif;
        }

const canvas = document.createElement('canvas'),
        context = canvas.getContext('2d'),
        width = canvas.width = window.innerWidth,
        height = canvas.height = window.innerHeight,
        HalfPI = Math.PI / 2,
        gravity = vector.create(0, 0.35),
        year = document.getElementById('year'),
        timeleft = document.getElementById('timeleft'),
        newyear = new Date('01/01/2020');
 
    let objects = [],
        startFireworks = false,
        newYearAlready = false;
 
    function renderTimeLeft() {
        let date = new Date();
 
        let delta = Math.abs(newyear - date) / 1000;
 
        let hours = Math.floor(delta / 3600) % 24;
        delta -= hours * 3600;
 
        let minutes = Math.floor(delta / 60) % 60;
        delta -= minutes * 60;
 
        let seconds = Math.floor(delta % 60) + 1;
 
        let string = '';
 
        let DaysLeft = Math.floor((newyear - date) / (1000 * 60 * 60 * 24)),
            HoursLeft = `${hours} ${hours > 1 ? 'hours' : 'hour'}`,
            MinutesLeft = `${minutes} ${minutes > 1 ? 'minutes' : 'minute'}`,
            SecondsLeft = `${seconds} ${seconds > 1 ? 'seconds' : 'second'}`;
 
        if (hours > 0) string = `${HoursLeft} & ${MinutesLeft}`;else
        if (minutes > 0) string = `${MinutesLeft} & ${SecondsLeft}`;else
            string = `${SecondsLeft}`;
 
        if (DaysLeft > 0) string = DaysLeft + ' days, ' + string;
        string += ' until 2020';
 
        timeleft.innerHTML = string;
    }
 
    renderTimeLeft();
 
    setInterval(function () {
        let date = new Date();
        if (date >= newyear) {
            if (!newYearAlready) {
                year.innerHTML = '2022';
                startFireworks = true;
                timeleft.innerHTML = 'Happy New Year!';
            }
 
            newYearAlready = true;
        } else renderTimeLeft();
    }, 500);
 
 
    document.body.appendChild(canvas);
 
    function random255() {
        return Math.floor(Math.random() * 100 + 155);
    }
 
    function randomColor() {
        let r = random255(),
            g = random255(),
            b = random255();
        return `rgb(${r}, ${g}, ${b})`;
    }
 
    class PhysicsBody {
        constructor() {
            objects.push(this);
        }
        PhysicsUpdate() {
            this.lastPosition = this.position.duplicate();
            this.position.addTo(this.velocity);
            this.velocity.addTo(gravity);
        }
        deleteObject() {
            objects[objects.indexOf(this)] = undefined;
        }}
 
 
    class firework extends PhysicsBody {
        constructor() {
            super();
            this.position = vector.create(Math.random() * width, height);
 
            let Velocity = vector.create(0, 0);
            Velocity.setLength(Math.random() * 10 + 15);
            Velocity.setAngle(Math.PI * 1.35 + Math.random() * Math.PI * 0.30);
            this.velocity = Velocity;
 
            this.trail = Math.floor(Math.random() * 4) != 1;
            this.trailColor = this.trail ? randomColor() : undefined;
            this.trailWidth = 2;
 
            this.TimeCreated = new Date().getTime();
            this.TimeExpired = this.TimeCreated + (Math.random() * 5 + 7) * 100;
 
            this.BlastParticleCount = Math.floor(Math.random() * 50) + 25;
            this.funky = Math.floor(Math.random() * 5) == 1;
 
            this.exposionColor = randomColor();
        }
 
        draw() {
            context.strokeStyle = this.trailColor;
            context.lineWidth = this.trailWidth;
 
            let p = this.position,
                lp = this.lastPosition;
 
            context.beginPath();
            context.moveTo(lp.getX(), lp.getY());
            context.lineTo(p.getX(), p.getY());
            context.stroke();
        }
 
        funkyfire() {
            var funky = this.funky;
            for (var i = 0; i < Math.floor(Math.random() * 10); i++) {
                new BlastParticle({ firework: this, funky });
            }
        }
 
        explode() {
            var funky = this.funky;
            for (var i = 0; i < this.BlastParticleCount; i++) {
                new BlastParticle({ firework: this, funky });
            }
            this.deleteObject();
        }
 
        checkExpire() {
            let now = new Date().getTime();
            if (now >= this.TimeExpired) this.explode();
        }
 
        render() {
            if (this.trail) this.draw();
            if (this.funky) this.funkyfire();
            this.checkExpire();
        }}
 
 
    class BlastParticle extends PhysicsBody {
        constructor({ firework, funky }) {
            super();
            this.position = firework.position.duplicate();
 
            let Velocity = vector.create(0, 0);
            if (!this.funky) {
                Velocity.setLength(Math.random() * 6 + 2);
                Velocity.setAngle(Math.random() * Math.PI * 2);
            } else {
                Velocity.setLength(Math.random() * 3 + 1);
                Velocity.setAngle(firework.getAngle + Math.PI / 2 - Math.PI * 0.25 + Math.PI * .5);
            }
 
            this.velocity = Velocity;
 
            this.color = firework.exposionColor;
 
            this.particleSize = Math.random() * 4;
 
            this.TimeCreated = new Date().getTime();
            this.TimeExpired = this.TimeCreated + (Math.random() * 4 + 3.5) * 100;
        }
 
        draw() {
            context.strokeStyle = this.color;
            context.lineWidth = this.particleSize;
            let p = this.position,
                lp = this.lastPosition;
 
            context.beginPath();
            context.moveTo(lp.getX(), lp.getY());
            context.lineTo(p.getX(), p.getY());
            context.stroke();
        }
 
        checkExpire() {
            let now = new Date().getTime();
            if (now >= this.TimeExpired) this.deleteObject();
        }
 
        render() {
            this.draw();
            this.checkExpire();
        }}
 
 
    document.body.addEventListener('mousedown', function (e) {
        let color = randomColor();
        for (var i = 0; i < Math.floor(Math.random() * 20) + 25; i++) {
            new BlastParticle({
                firework: {
                    position: vector.create(e.pageX, e.pageY),
                    velocity: vector.create(0, 0),
                    exposionColor: color },
 
                funky: false });
 
        }
    });
 
    setInterval(function () {
        if (!startFireworks) return;
        for (var i = 0; i < Math.floor(Math.random() * 4); i++) {
            new firework();
        }
    }, 500);
 
    function cleanupObjects() {
        objects = objects.filter(o => o != undefined);
    }
 
    function loop() {
        context.fillStyle = 'rgba(0,0,0,0.085)';
        context.fillRect(0, 0, width, height);
 
        let unusedObjectCount = 0;
        objects.map(o => {
            if (!o) {unusedObjectCount++;return;}
            o.PhysicsUpdate();
            o.render();
        });
        if (unusedObjectCount > 100) cleanupObjects();
 
        requestAnimationFrame(loop);
    }
 
    loop();