function write_fire(e) {
    var t, n, r;
    stars[e + "r"] = createDiv("|", 12);
    boddie.appendChild(stars[e + "r"]);
    for (t = bits * e; t < bits + bits * e; t++) {
        stars[t] = createDiv("*", 13);
        boddie.appendChild(stars[t])
    }
}

function createDiv(e, t) {
    var n = document.createElement("div");
    n.style.font = t + "px monospace";
    n.style.position = "absolute";
    n.style.backgroundColor = "transparent";
    n.appendChild(document.createTextNode(e));
    return n
}

function launch(e) {
    colour[e] = Math.floor(Math.random() * colours.length);
    Xpos[e + "r"] = swide * .5;
    Ypos[e + "r"] = shigh - 5;
    bangheight[e] = Math.round((.5 + Math.random()) * shigh * .4);
    dX[e + "r"] = (Math.random() - .5) * swide / bangheight[e];
    if (dX[e + "r"] > 1.25) stars[e + "r"].firstChild.nodeValue = "/";
    else if (dX[e + "r"] < -1.25) stars[e + "r"].firstChild.nodeValue = "\\";
    else stars[e + "r"].firstChild.nodeValue = "|";
    stars[e + "r"].style.color = colours[colour[e]]
}

function bang(e) {
    var t, n, r = 0;
    for (t = bits * e; t < bits + bits * e; t++) {
        n = stars[t].style;
        n.left = Xpos[t] + "px";
        n.top = Ypos[t] + "px";
        if (decay[t]) decay[t]--;
        else r++;
        if (decay[t] == 15) n.fontSize = "7px";
        else if (decay[t] == 7) n.fontSize = "2px";
        else if (decay[t] == 1) n.visibility = "hidden";
        Xpos[t] += dX[t];
        Ypos[t] += dY[t] += 1.25 / intensity[e]
    }
    if (r != bits) setTimeout("bang(" + e + ")", speed)
}

function stepthrough(e) {
    var t, n, r;
    var i = Xpos[e + "r"];
    var s = Ypos[e + "r"];
    Xpos[e + "r"] += dX[e + "r"];
    Ypos[e + "r"] -= 4;
    if (Ypos[e + "r"] < bangheight[e]) {
        n = Math.floor(Math.random() * 3 * colours.length);
        intensity[e] = 5 + Math.random() * 4;
        for (t = e * bits; t < bits + bits * e; t++) {
            Xpos[t] = Xpos[e + "r"];
            Ypos[t] = Ypos[e + "r"];
            dY[t] = (Math.random() - .5) * intensity[e];
            dX[t] = (Math.random() - .5) * (intensity[e] - Math.abs(dY[t])) * 1.25;
            decay[t] = 16 + Math.floor(Math.random() * 16);
            r = stars[t];
            if (n < colours.length) r.style.color = colours[t % 2 ? colour[e] : n];
            else if (n < 2 * colours.length) r.style.color = colours[colour[e]];
            else r.style.color = colours[t % colours.length];
            r.style.fontSize = "13px";
            r.style.visibility = "visible"
        }
        bang(e);
        launch(e)
    }
    stars[e + "r"].style.left = i + "px";
    stars[e + "r"].style.top = s + "px"
}

function set_width() {
    var e = 999999;
    var t = 999999;
    if (document.documentElement && document.documentElement.clientWidth) {
        if (document.documentElement.clientWidth > 0) e = document.documentElement.clientWidth;
        if (document.documentElement.clientHeight > 0) t = document.documentElement.clientHeight
    }
    if (typeof self.innerWidth != "undefined" && self.innerWidth) {
        if (self.innerWidth > 0 && self.innerWidth < e) e = self.innerWidth;
        if (self.innerHeight > 0 && self.innerHeight < t) t = self.innerHeight
    }
    if (document.body.clientWidth) {
        if (document.body.clientWidth > 0 && document.body.clientWidth < e) e = document.body.clientWidth;
        if (document.body.clientHeight > 0 && document.body.clientHeight < t) t = document.body.clientHeight
    }
    if (e == 999999 || t == 999999) {
        e = 800;
        t = 600
    }
    swide = e;
    shigh = t
}
var bits = 80;
var speed = 32;
var bangs = 5;
var colours = new Array("#03f", "#f03", "#0e0", "#93f", "#0cf", "#f93", "#f0c");
var bangheight = new Array;
var intensity = new Array;
var colour = new Array;
var Xpos = new Array;
var Ypos = new Array;
var dX = new Array;
var dY = new Array;
var stars = new Array;
var decay = new Array;
var swide = 800;
var shigh = 600;
var boddie;
function run_snow() {
console.log('run');
    if (document.getElementById) {
        var e;
        boddie = document.createElement("div");
        boddie.id = 'div_snow';
        boddie.style.position = "fixed";
        boddie.style.top = "0px";
        boddie.style.left = "0px";
        boddie.style.overflow = "visible";
        boddie.style.width = "1px";
        boddie.style.height = "1px";
        boddie.style.backgroundColor = "transparent";
        document.body.appendChild(boddie);
        set_width();
        for (e = 0; e < bangs; e++) {
            write_fire(e);
            launch(e);
            setInterval("stepthrough(" + e + ")", speed)
        }
    }
}
window.onresize = set_width();
run_snow();

<html>
<head>
<title>snow</title>
</head>
<body>
<script>
function loadSnow(){
    let script = document.createElement('script');
    script.type = 'text/javascript';
    script.src = 'snow.js';
    document.getElementsByTagName('body')[0].appendChild(script);
}
function unloadSnow(){
    let body = document.getElementsByTagName('body')[0];
    let script = document.querySelector('script[src="snow.js"]');
    body.removeChild(script);
    
    let div_snow = document.getElementById('div_snow');
    body.removeChild(div_snow);
}
</script>
<button onclick='loadSnow()'>Let it snow</button>
<button onclick='unloadSnow()'>Now snow</button>
</body>
</html>