save D3 as SVG

d3tosvg.js

const oReq = new XMLHttpRequest();
oReq.open("POST", 'http://localhost:3000/upload', true);
oReq.onload = function (oEvent) {
  console.log('uploaded!');
  // File was Uploaded ok.
};
// var html = d3.select("svg").node().parentNode.innerHTML;
const svgDoctype = '<?xml version="1.0" standalone="no"?>'
  + '<!DOCTYPE svg PUBLIC "-//W3C//DTD SVG 1.1//EN" "http://www.w3.org/Graphics/SVG/1.1/DTD/svg11.dtd">';

// serialize our SVG XML to a string.
let svgString = (new XMLSerializer()).serializeToString(d3.select('svg').node());

// RegEx to round the path coordinates (1.146994119497445 to 1.1, or if using toFixed(2), to 1.15) in order to trim the file size by a lot
svgString = svgString.replace(/([\-+]?\d{1,}\.\d{3,}([eE][\-+]?\d+)?)/g, function(x){ return (+x).toFixed(1)  });

const blob = new Blob([ svgDoctype + svgString], { type: 'image/svg+xml;charset=utf-8' });

oReq.send(blob);

server.js

const bodyParser = require('body-parser');

app.use(bodyParser.raw({ type: 'image/svg+xml', limit: '50mb' }))

app.use(function (req, res, next) {

    // Website you wish to allow to connect
    res.setHeader('Access-Control-Allow-Origin', '*');

    // Request methods you wish to allow
    res.setHeader('Access-Control-Allow-Methods', 'GET, POST, OPTIONS, PUT, PATCH, DELETE');

    // Request headers you wish to allow
    res.setHeader('Access-Control-Allow-Headers', 'X-Requested-With,content-type');

    // Set to true if you need the website to include cookies in the requests sent
    // to the API (e.g. in case you use sessions)
    res.setHeader('Access-Control-Allow-Credentials', true);

    // Pass to next layer of middleware
    next();
});

app.post('/upload', function(req, res) {
    var fs = require('fs');
    //console.log(req.body);

    fs.writeFile('public/uploads/' + Date.now() + ".svg", req.body, 'binary', function (err){
        if(err) {
            return console.log(err);
        }
        console.log("The file was saved!");
        res.send('File uploaded!');
    });
});