astro/packages/integrations/node/src/createOutgoingHttpHeaders.ts

import type { OutgoingHttpHeaders } from 'node:http';

/**
 * Takes in a nullable WebAPI Headers object and produces a NodeJS OutgoingHttpHeaders object suitable for usage
 * with ServerResponse.writeHead(..) or ServerResponse.setHeader(..)
 *
 * @param webHeaders WebAPI Headers object
 * @returns NodeJS OutgoingHttpHeaders object with multiple set-cookie handled as an array of values
 */
export const createOutgoingHttpHeaders = (
	webHeaders: Headers | undefined | null
): OutgoingHttpHeaders | undefined => {
	if (!webHeaders) {
		return undefined;
	}

	// re-type to access Header.getSetCookie()
	const headers = webHeaders as HeadersWithGetSetCookie;

	// at this point, a multi-value'd set-cookie header is invalid (it was concatenated as a single CSV, which is not valid for set-cookie)
	const nodeHeaders: OutgoingHttpHeaders = Object.fromEntries(headers.entries());

	if (Object.keys(nodeHeaders).length === 0) {
		return undefined;
	}

	// if there is > 1 set-cookie header, we have to fix it to be an array of values
	if (headers.has('set-cookie')) {
		const cookieHeaders = headers.getSetCookie();
		if (cookieHeaders.length > 1) {
			// the Headers.entries() API already normalized all header names to lower case so we can safely index this as 'set-cookie'
			nodeHeaders['set-cookie'] = cookieHeaders;
		}
	}

	return nodeHeaders;
};

interface HeadersWithGetSetCookie extends Headers {
	// the @astrojs/webapi polyfill makes this available (as of undici@5.19.0), but tsc doesn't pick it up on the built-in Headers type from DOM lib
	getSetCookie(): string[];
}