xml2js parse and build round trip: working versions (tested 2026-10-16)

{
  "name": "xml2js-parse-build",
  "private": true,
  "type": "module",
  "scripts": {
    "test": "node --import tsx --test test/*.test.ts"
  },
  "dependencies": {
    "xml2js": "0.6.2"
  },
  "devDependencies": {
    "@types/node": "26.6.5",
    "@types/xml2js": "0.4.14",
    "tsx": "4.23.15"
  }
}

import { Builder, parseStringPromise } from "xml2js";

export type Book = {
  id: string;
  lang: string;
  title: string;
  author: string | string[];
};

export type Catalog = {
  catalog: {
    book: Book[];
  };
};

export const parserOptions = {
  explicitArray: false,
  mergeAttrs: true,
} as const;

export async function parseCatalog(xml: string): Promise<Catalog> {
  return (await parseStringPromise(xml, parserOptions)) as Catalog;
}

export function buildCatalog(catalog: Catalog): string {
  const builder = new Builder({ headless: true, renderOpts: { pretty: false } });
  const books = catalog.catalog.book.map(({ id, lang, ...rest }) => ({
    $: { id, lang },
    ...rest,
  }));
  return builder.buildObject({ catalog: { book: books } });
}

import { test } from "node:test";
import assert from "node:assert/strict";
import { parseCatalog, buildCatalog } from "../src/convert.ts";

const xml = `<?xml version="1.0" encoding="UTF-8"?>
<catalog>
  <book id="b1" lang="en">
    <title>Dune</title>
    <author>Frank Herbert</author>
    <author>Kevin J. Anderson</author>
  </book>
  <book id="b2" lang="fr">
    <title>Le Petit Prince</title>
    <author>Antoine de Saint-Exupery</author>
  </book>
</catalog>`;

test("parses attributes and repeated elements", async () => {
  const parsed = await parseCatalog(xml);
  assert.equal(parsed.catalog.book.length, 2);
  assert.equal(parsed.catalog.book[0].id, "b1");
  assert.equal(parsed.catalog.book[0].lang, "en");
  assert.equal(parsed.catalog.book[0].title, "Dune");
  assert.deepEqual(parsed.catalog.book[0].author, ["Frank Herbert", "Kevin J. Anderson"]);
  assert.equal(parsed.catalog.book[1].title, "Le Petit Prince");
  assert.equal(parsed.catalog.book[1].author, "Antoine de Saint-Exupery");
});

test("builds XML back from parsed data", async () => {
  const parsed = await parseCatalog(xml);
  const output = buildCatalog(parsed);
  assert.ok(output.includes("<book"));
  assert.ok(output.includes('id="b1"'));
  assert.ok(output.includes("<title>Dune</title>"));
  assert.ok(output.includes("<author>Frank Herbert</author>"));
  assert.ok(output.includes("<author>Kevin J. Anderson</author>"));
  assert.ok(output.includes("<title>Le Petit Prince</title>"));
  assert.equal((output.match(/<book/g) ?? []).length, 2);
});