fix: Replace with fast-xml-parser (pure ESM) which works in all JS runtimes including workerd. by maikunari · Pull Request #596 · emdash-cms/emdash

maikunari · 2026-04-16T06:53:15Z

The WordPress WXR parser used the 'sax' package (CommonJS-only) which caused 'module is not defined' errors in Cloudflare Workers (workerd).

Replace with fast-xml-parser (pure ESM) which works in all JS runtimes including workerd.

Key changes:

Replace SAX event-driven parsing with tree-based XML parsing
parseWxrString() and parseWxr() now use fast-xml-parser's XMLParser
Remove sax dependency, add fast-xml-parser@^5.6.0
Add 15 unit tests for the new parser implementation
Maintain identical output types and behavior (WxrData interface unchanged)

Fixes #573

What does this PR do?

Replaces the sax XML parser (CommonJS-only) with fast-xml-parser (pure ESM) so WordPress WXR imports work in Cloudflare Workers (workerd). The original sax dependency crashes with module is not defined in workerd.
Fixes #573

Closes #

Type of change

Checklist

I have read CONTRIBUTING.md
pnpm typecheck passes
pnpm lint passes
pnpm test passes (or targeted tests for my change)
pnpm format has been run
I have added/updated tests for my changes (if applicable)
User-visible strings in the admin UI are wrapped for translation and pnpm locale:extract has been run (if applicable)
I have added a changeset (if this PR changes a published package)
New features link to an approved Discussion: https://github.com/emdash-cms/emdash/discussions/...

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github-actions · 2026-04-16T06:53:26Z

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commit: e5d3236

ascorbic · 2026-04-16T08:11:49Z

How does memory use compare?

maikunari · 2026-04-16T08:18:30Z

How does memory use compare?

Fast-xml-parser builds a full object tree, so it does use more memory than SAX's streaming approach. However, sax is CJS-only and crashes on Cloudflare Workers with module is not defined, so it's not viable for the workerd runtime.

For typical WXR files (most are under 50MB), the memory footprint is reasonable. If very large imports become an issue down the line, we could explore chunked processing — but I figured getting imports working on Cloudflare first was the priority. Happy to optimize later if needed!

Copilot

Pull request overview

Replaces the WordPress WXR parser dependency from CommonJS-only sax to fast-xml-parser to avoid module is not defined failures in Cloudflare Workers (workerd), while keeping the existing WxrData output shape.

Changes:

Swap SAX event-driven parsing for fast-xml-parser tree-based parsing in the WXR parser.
Remove sax, add fast-xml-parser@^5.6.0 (and update lockfile).
Add a new unit test suite covering parseWxrString() scenarios.

Reviewed changes

Copilot reviewed 3 out of 4 changed files in this pull request and generated 9 comments.

File	Description
`packages/core/src/cli/wxr/parser.ts`	Reimplements WXR parsing using `fast-xml-parser`, including item/meta/taxonomy extraction and stream handling.
`packages/core/tests/unit/wxr/parser.test.ts`	Adds unit tests for `parseWxrString()` to validate behavior in non-Node runtimes.
`packages/core/package.json`	Replaces `sax` dependency with `fast-xml-parser`.
`pnpm-lock.yaml`	Lockfile updates reflecting dependency swap and transitive deps.

Files not reviewed (1)

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+const SINGLE_POST_WXR = `<?xml version="1.0" encoding="UTF-8"?>
+<rss version="2.0"
+	xmlns:content="http://purl.org/rss/1.0/modules/content/"
+	xmlns:dc="http://purl.org/dc/elements/1.1/"


+		it("parses a single published post with categories, tags, and meta", async () => {
+			const data = await parseWxrString(SINGLE_POST_WXR);
+
+			expect(data.posts.length).toBe(1);
+			const post = data.posts[0];


+
+	post.title = getText(item["title"]) || undefined;
+	post.link = getText(item["link"]) || undefined;
+	post.pubDate = getText(item["pubdate"]) || undefined;


+		stream.on("data", (chunk: Buffer) => {
+			chunks.push(chunk);


+/** Parse a numeric string, returning undefined for NaN/0/missing */
+function parseIntSafe(val: string | undefined): number | undefined {
+	if (!val) return undefined;
+	const n = parseInt(val, 10);
+	return isNaN(n) ? undefined : n;


+ * This is compatible with all environments; for very large files (>100MB),
+ * consider using parseWxrString() with chunked reading instead.


+
+import { describe, it, expect } from "vitest";
+
+import { parseWxrString, type WxrData } from "../../../src/cli/wxr/parser.js";


    "citty": "^0.1.6",
    "consola": "^3.4.2",
    "croner": "^10.0.1",
+    "fast-xml-parser": "^5.6.0",


+	const parser = createWxrParser();
+	const parsed = parser.parse(xml) as Record<string, unknown>;
+	return Promise.resolve(extractWxrData(parsed));


The WordPress WXR parser used the 'sax' package (CommonJS-only) which caused 'module is not defined' errors in Cloudflare Workers (workerd). Replace with fast-xml-parser (pure ESM, zero deps) which works in all JS runtimes including workerd. Key changes: - Replace SAX event-driven parsing with tree-based XML parsing - parseWxrString() and parseWxr() now use fast-xml-parser's XMLParser - Remove sax dependency, add fast-xml-parser@^5.6.0 - Add 15 unit tests for the new parser implementation - Maintain identical output types and behavior (WxrData interface unchanged) Fixes emdash-cms#573

ascorbic · 2026-04-16T10:03:52Z

For handling CJS, our approach is usually to add the dependency to optimizeDeps in packages/core/src/astro/integration/vite-config.ts. We have had users who have raised concerns about memory usage in imports, even with the current approach, so I don't think it's viable to move to something fully in-memory.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

maikunari · 2026-04-16T10:13:12Z

Addressed Copilot's review feedback:

Fixed pubDate casing bug (was reading lowercase, WXR uses capital D)
Fixed stream encoding handling to accept both Buffer and string chunks
Wrapped parseWxrString in try/catch for consistent Promise rejection
Added namespace declaration to test fixture and pubDate assertion (verified this assertion fails against the pre-fix code)
Cleaned up docstrings and removed unused import

The original sax → fast-xml-parser commit introduced 52 TS2345 errors that weren't caught locally because typecheck requires workspace builds. The helpers declared `XmlNode = string | number | ... | undefined` but every call site passes values from `Record<string, unknown>` indexing. Widen the helper signatures to accept `unknown`. The helpers already defensively runtime-check every branch, so permissive input types are safe. Removed the now-unused `XmlNode` type alias. Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

maikunari · 2026-04-16T10:21:30Z

Follow-up: the typecheck failure in CI turned out to be pre-existing on the branch — the original sax → fast-xml-parser commit introduced 52 TS2345 errors that weren't caught locally because typecheck requires workspace builds. Widened getText/getAttr to accept unknown (they already runtime-check every branch, so permissive input is safe). All checks should pass now.

After widening node to unknown, two paths could call String() on arbitrary values, producing "[object Object]" or similar. Replace the fallback with "" and narrow obj["#text"] to string|number before stringifying. Matches existing defensive pattern for missing #text. Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

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maikunari and others added 2 commits April 16, 2026 19:02

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fix(wxr): address review feedback from emdash-cms#596

ec075f7

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

maikunari force-pushed the fix/wxr-cloudflare-compat branch from 8f2688f to ec075f7 Compare April 16, 2026 10:10

maikunari changed the title ~~fix: replace sax with fast-xml-parser for Cloudflare Workers compat~~ fix: Replace with fast-xml-parser (pure ESM) which works in all JS runtimes including workerd. Apr 16, 2026

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fix: Replace with fast-xml-parser (pure ESM) which works in all JS runtimes including workerd.#596

fix: Replace with fast-xml-parser (pure ESM) which works in all JS runtimes including workerd.#596
maikunari wants to merge 5 commits intoemdash-cms:mainfrom
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		* This is compatible with all environments; for very large files (>100MB),
		* consider using parseWxrString() with chunked reading instead.


		import { describe, it, expect } from "vitest";

		import { parseWxrString, type WxrData } from "../../../src/cli/wxr/parser.js";

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