@sdxc/atom

Atom 1.0 feed parser and builder.

It reads RFC 4287 documents into an Atom instance and writes that instance back as XML. Every link keeps its relation, dates stay the text the source held, and authors stay where the document put them, so the consumer picks the precedence it wants. Elements outside the Atom namespace are kept as extensions and survive a round trip.

Installation

npm add @sdxc/atom

Parsing answers a Result from @sdxc/result, and Atom.fromXML reads a document from @sdxc/xml. Both install alongside this package.

Usage

Parse a feed

import { Atom } from "@sdxc/atom";
import { isFailure } from "@sdxc/result";

let result = Atom.parse(xml, "https://example.com/feed.xml");
if (isFailure(result)) throw result.error;

let atom = result.data;
atom.feed.title; // "Example Blog"
for (let entry of atom.entries) console.log(entry.id, entry.updated);

The second argument is the document's own URL; relative href, uri, src, icon and logo values resolve against it and any xml:base the document declares.

Fetch a feed

import { Atom } from "@sdxc/atom";
import { isFailure } from "@sdxc/result";

let result = await Atom.fetch(new URL("https://example.com/feed.xml"));
if (isFailure(result)) return result.error.message;

Any Content-Type is accepted, and relative references resolve against the URL the response finally came from, so a redirected feed still yields absolute links.

Build a feed

import { Atom } from "@sdxc/atom";

let atom = new Atom({
	id: "tag:example.com,2026:feed",
	title: "Example Blog",
	updated: new Date().toISOString(),
	link: [
		{ href: "https://example.com/", rel: "alternate", type: "text/html" },
		{ href: "https://example.com/feed.xml", rel: "self", type: "application/atom+xml" },
	],
	author: { name: "Ada Lovelace" },
});

atom.addEntry({
	id: "tag:example.com,2026:post-1",
	title: "Hello World",
	updated: "2026-04-14T10:30:00Z",
	published: "2026-04-14T09:00:00Z",
	summary: "A short summary",
	content: { type: "html", value: "<p>Full post content</p>" },
	link: { href: "https://example.com/posts/hello", rel: "alternate" },
});

let xml = atom.toString();

The feed and every entry require id, title and updated; the constructor and addEntry throw an AtomParseError naming the first one missing.

API

new Atom(feed: Atom.Feed)

Creates a feed from its metadata with no entries. Throws AtomParseError when id, title or updated is missing.

atom.feed

The feed-level metadata, as a clone. Assigning a new Atom.Feed validates and replaces it, leaving the entries in place.

atom.entries

The entries, as clones, in the order they were added.

atom.addEntry(entry: Atom.Entry): void

Appends one entry. Throws AtomParseError when id, title or updated is missing.

atom.removeEntry(id: string): void

Removes the entry carrying id; an unknown id leaves the feed unchanged.

atom.toJSON(): Atom.Document

Returns { feed, entries } as plain serializable data.

atom.toString(): string

Serializes the feed to Atom 1.0 XML with the Atom namespace as the default. Throws AtomStringifyError when the data cannot form a valid document.

Atom.parse(source: string, base?: string): Result<Atom, AtomParseError>

Parses XML text, resolving relative references against base.

Atom.fromXML(xml: XML, base?: string): Result<Atom, AtomParseError>

Reads a feed out of a document already parsed with XML.parse.

Atom.fetch(input: URL | RequestInfo, init?: RequestInit): Promise<Result<Atom, AtomFetchError | AtomParseError>>

Retrieves a document with the global fetch and parses it. A rejected request or a non-2xx status answers AtomFetchError; an unparseable body answers AtomParseError.

Errors

• AtomParseError: the document is not an Atom feed, or metadata lacks a required field.

• AtomFetchError: the request failed or answered with an error status.

• AtomStringifyError: the feed cannot be written as XML.

Types

All types live on the Atom namespace. A field that may repeat holds one value when the document had one and an array when it had several.

• Atom.Feed: feed metadata. id, title and updated are required; subtitle, rights, author, contributor, link, category, generator, icon and logo are optional. lang and base hold the root's xml:lang and xml:base, namespaces its prefix declarations, and attributes any other root attributes.

• Atom.Entry: one entry. id, title and updated are required; it adds published, summary, content and source, plus its own lang and base when they differ from the feed's.

• Atom.Text / Atom.TextInput: a text construct, { value, type? } with type of text, html or xhtml, or a bare string for plain text.

• Atom.Content: an entry body with any media type, an inline value, or a src for a body held elsewhere.

• Atom.Person (name, uri?, email?), Atom.Link (href, rel?, type?, hreflang?, title?, length?), Atom.Category (term, scheme?, label?), Atom.Generator (value, uri?, version?) and Atom.Source, the metadata of the feed an entry was copied from.

• Atom.PersonInput, Atom.LinkInput, Atom.CategoryInput: one value or an array; a category may be a bare term string.

• Atom.Element / Atom.Node: a foreign element { name, attributes?, children? }, kept in the extensions array of a feed, entry, person, link, category or source.

• Atom.Document: { feed, entries }, the shape toJSON returns.

Pattern: Reading Entries For Display

Links keep rel absent when the document left it out, text constructs arrive as a string or { value, type }, dates are the raw RFC 3339 text, and an entry's author may sit on the entry, its source or the feed. A reader applies the defaults it wants at the point of use.

import { Atom } from "@sdxc/atom";
import { isFailure } from "@sdxc/result";

function list<T>(value: T | T[] | undefined): T[] {
	if (value === undefined) return [];
	return Array.isArray(value) ? value : [value];
}

function text(value: Atom.TextInput | undefined): string {
	if (value === undefined) return "";
	return typeof value === "string" ? value : value.value;
}

let result = await Atom.fetch(new URL("https://example.com/feed.xml"));
if (isFailure(result)) throw result.error;

let atom = result.data;
let items = atom.entries.map((entry) => {
	let page = list(entry.link).find((link) => (link.rel ?? "alternate") === "alternate");
	let author = list(entry.author ?? entry.source?.author ?? atom.feed.author)[0];
	return {
		title: text(entry.title),
		url: page?.href,
		author: author?.name,
		publishedAt: new Date(entry.published ?? entry.updated),
	};
});

An html or xhtml value is the publisher's markup, unsanitized; escape or sanitize it before rendering.

Pattern: Serving A Feed From A Request Handler

Build the feed from your records and answer it with the Atom media type.

import { Atom } from "@sdxc/atom";

interface Post {
	slug: string;
	title: string;
	excerpt: string;
	html: string;
	publishedAt: Date;
	updatedAt: Date;
}

function feedResponse(posts: Post[]): Response {
	let atom = new Atom({
		id: "https://example.com/",
		title: "Example Blog",
		updated: (posts[0]?.updatedAt ?? new Date()).toISOString(),
		link: { href: "https://example.com/feed.xml", rel: "self" },
		author: { name: "Ada Lovelace", uri: "https://example.com/about" },
		generator: { value: "Example CMS", version: "1.0" },
	});

	for (let post of posts) {
		atom.addEntry({
			id: `https://example.com/posts/${post.slug}`,
			title: post.title,
			updated: post.updatedAt.toISOString(),
			published: post.publishedAt.toISOString(),
			summary: post.excerpt,
			content: { type: "html", value: post.html },
			link: { href: `https://example.com/posts/${post.slug}` },
			category: ["writing"],
		});
	}

	return new Response(atom.toString(), {
		headers: { "Content-Type": "application/atom+xml; charset=utf-8" },
	});
}

Pattern: Editing A Feed In Place

Parse an existing document, change its entries and metadata, and write it back. Everything the methods leave alone, foreign elements included, comes out as it went in.

import { Atom } from "@sdxc/atom";
import { isFailure } from "@sdxc/result";

let result = Atom.parse(xml, "https://example.com/feed.xml");
if (isFailure(result)) throw result.error;

let atom = result.data;
atom.removeEntry("tag:example.com,2026:post-1");
atom.addEntry({
	id: "tag:example.com,2026:post-2",
	title: { value: "Second <em>post</em>", type: "html" },
	updated: "2026-05-01T08:00:00Z",
});
atom.feed = { ...atom.feed, updated: "2026-05-01T08:00:00Z" };

let updated = atom.toString();

An xhtml text construct or body is written back as html, carrying the same markup.

Pattern: Foreign Elements And Namespaces

An element counts as Atom when its prefix resolves to http://www.w3.org/2005/Atom, whatever the prefix is; every other element lands in extensions with its qualified name. To write one, declare its prefix in the feed's namespaces.

import { Atom } from "@sdxc/atom";
import { isFailure } from "@sdxc/result";

let atom = new Atom({
	id: "tag:example.com,2026:podcast",
	title: "Example Podcast",
	updated: "2026-04-14T10:30:00Z",
	namespaces: { media: "http://search.yahoo.com/mrss/" },
	extensions: [
		{ name: "media:rating", attributes: { scheme: "urn:simple" }, children: ["nonadult"] },
	],
});

atom.addEntry({
	id: "tag:example.com,2026:episode-1",
	title: "Episode 1",
	updated: "2026-04-14T10:30:00Z",
	link: {
		href: "https://example.com/episode-1.mp3",
		rel: "enclosure",
		type: "audio/mpeg",
		length: 12345678,
	},
	extensions: [{ name: "media:thumbnail", attributes: { url: "https://example.com/1.jpg" } }],
});

let result = Atom.parse(atom.toString());
if (isFailure(result)) throw result.error;

result.data.feed.extensions;
// [{ name: "media:rating", attributes: { scheme: "urn:simple" }, children: ["nonadult"] }]

Pattern: Parsing An Already-Parsed Document

When the XML is parsed for another purpose first, such as sniffing whether it is Atom or RSS, hand the document to Atom.fromXML instead of parsing the text twice.

import { Atom } from "@sdxc/atom";
import { isFailure } from "@sdxc/result";
import { XML } from "@sdxc/xml";

let parsed = XML.parse(text);
if (isFailure(parsed)) throw parsed.error;

let result = Atom.fromXML(parsed.data, "https://example.com/feed.xml");
if (isFailure(result)) {
	// not an Atom feed: try another format with parsed.data
}