How do I save an svg node to a file using nodejs?

I'm using d3 through a small helper package: https://www.npmjs.com/package/d3-node

I created my svg file using d3-node, here's the code.

const D3Node = require('d3-node')

var d3n = new D3Node()
var svg = d3n.createSVG()
    .style("width","1920px")
    .style("height","1080px") 
    .attr("preserveAspectRatio","true")
.html(firstTemplate)
.append("myCustomTag")

Now after this, I have no idea how to save the output. The main problem is in myCustomTag.

console.log(d3n.d3Element.select("svg").node().innerHTML)

This line is supposed to output my svg, and it does, except that myCustomTag becomes mycustomtagand my svg gets corrupt.

I have tried select("svg").node().outerHTML, select("svg").html(), innerText, I just couldn't find anything.

I can't use the innerHTML in this case, is there a way to store the svg file directly from the d3 variable?

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