Exporting an SVG from Figma and identifying which SVG elements in the exported file correspond to which objects in Figma

example.js

const getSvgNamesForNodes = () => {
    // this is guesswork. When Figma has two nodes with the same name (say, "Union"),
    // it serialises them into SVG with names "Union" and "Union_2", but it's not
    // clear which one becomes "Union" and which "Union_2". It is theorised that this
    // is done in the same order that .findAll returns, so we make use of that.
    // Figma might decide to change this at any time, of course. It would be much
    // nicer if there were an SVG export option which also serialised getPluginData()
    // data as data-* attributes in the output SVG, but there isn't, yet.
    let names = {};
    let nameIndices = {};
    window.figma.currentPage.findAll().forEach(o => {
        let index = "";
        if (nameIndices[o.name]) {
            index = "_" + (nameIndices[o.name] + 1);
            nameIndices[o.name] += 1
        } else {
            nameIndices[o.name] = 1;
        }
        names[o.name + index] = o;
    });
    return names;
}

// grab the current page as an SVG
const im = await figma.currentPage.exportAsync({format:"SVG", svgIdAttribute: true});
const svgtext = new TextDecoder("utf-8").decode(im);
// parse it into a DOM
const parser = new DOMParser();
const doc = parser.parseFromString(svgtext, "image/svg+xml");
// get the calculated names for each Figma node
const mapping = getSvgNamesForNodes();
// use the names to pair each SVG element with its corresponding Figma node, and copy some info across
for (let calculated_svgname in mapping) {
    let svgel = doc.getElementById(calculated_svgname);
    if (!svgel) { continue; }
    svgel.setAttribute("data-figma-nodeid", mapping[calculated_svgname].id);
}
// there should now be no SVG elements with an ID but without a nodeid: check, for paranoia
const unmatched = doc.querySelectorAll("[id]:not([data-figma-nodeid])");
if (unmatched.length > 0) {
    console.warn("Failed to match up the following nodes:", unmatched);
}
// serialise the DOM back into svg text
const finaltext = new XMLSerializer().serializeToString(doc);
console.log(finaltext);