It converts the SVG file into base64 encoded form to include images fed from external sources and link files such as css and fonts. (I created this so that when a download request is sent with "Export the board" in miro.com and "Save as PDF", the svg file in the "Request Payload" section transmitted from the Developer Console > Network is given …

svgExternalSourcesBase64Converter.js

const axios = require('axios');
const fs = require('fs');

// SVG file to convert
const filePath = './svg_external.svg';
// Output file path
const outputFilePath = './svg_external_export_base64.svg';


/**
 * SVG External URL Fetcher
 * It allows you to find the external URL in the SVG content.
 * @param {*} svg SVG content
 * @returns {Array} external URLs
 */
const urlFinder = async (svg) => {
	const urlRegex = /url\("(.*?)"\)/g;
	const matches = svg.match(urlRegex);
	return matches;
};


/**
 * SVG External Image Fetcher
 * It allows you to find the image tag in the SVG content.
 * @param {*} svg SVG content
 * @returns {Array} image tags
 */
const imageFinder = async (svg) => {
	const imageRegex = /<image.*?xlink:href="(.*?)".*?>/g;
	const matches = svg.match(imageRegex);
	return matches;
};

/**
 * External Source Fetcher
 * It allows you to send requests to external source connections and get output as base64.
 * @param {*} url any external URL
 * @returns base64
 */
const externalSourceFetcher = async (url) => {
	console.info(`Fetching data from ${url}`);
	const response = await axios.get(url, {
		timeout: 10000,
		redirect: 'follow',
		responseType: 'arraybuffer'
	}).catch((error) => {
		console.error('externalSourceFetcher Error:',error?.response?.status, error?.response?.statusText);
	});

	let headers = response.headers;
	let contentType = headers['content-type'];
	if (!contentType) {
		console.error('No content type found', url);
		return;
	}
	// Base64 encoding
	let base64 = Buffer.from(response.data, 'binary').toString('base64');
	base64 = `data:${contentType};base64,${base64}`;
	return base64;
};

// Runner
(async () => {
	// Read SVG file
	fs.readFile(filePath, 'utf8', async (err, data) => {
		if (err) {
			console.error(err);
			return;
		}
		let svg = data;

		// External Images
		const images = await imageFinder(svg);
		for (let i = 0; i < images.length; i++) {
			const image = images[i];
			const imageUrl = image.match(/xlink:href="(.*?)"/)[1];
			if (!imageUrl) {
				continue;
			}
			if (!imageUrl.includes('http')) {
				continue;
			}
			if (imageUrl.includes('data:image/svg+xml')) {
				continue;
			}
			const imageSvg = await externalSourceFetcher(imageUrl);
			svg = svg.replace(imageUrl, imageSvg);
		}

		// External URLs
		const urls = await urlFinder(svg);
		for (let i = 0; i < urls.length; i++) {
			const url = urls[i];
			const extUrl = url.match(/url\("(.*?)"\)/)[1];
			if (!extUrl) {
				continue;
			}
			if (!extUrl.includes('http')) {
				continue;
			}
			const imageSvg = await externalSourceFetcher(extUrl);
			svg = svg.replace(extUrl, imageSvg);
		}

		// Write SVG file
		fs.writeFile(outputFilePath, svg, (err) => {
			if (err) {
				console.error('SVG File Write Error:',err);
				return;
			}
			console.log('SVG file created');
		});
	});
})();

Info:

It converts the SVG file into base64 encoded form to include images fed from external sources and link files such as css and fonts.

I created this so that when a download request is sent with Export the board in miro.com and Save as PDF, the svg file in the Request Payload section transmitted from the Developer Console > Network is given to the software and the external resources of the svg are converted to base64.