unpkg/modules/middleware/fetchPackage.js

import semver from 'semver';

import addLeadingSlash from '../utils/addLeadingSlash.js';
import createPackageURL from '../utils/createPackageURL.js';
import createSearch from '../utils/createSearch.js';
import { getPackageInfo as getNpmPackageInfo } from '../utils/npm.js';

function tagRedirect(req, res) {
  const version = req.packageInfo['dist-tags'][req.packageVersion];

  res
    .set({
      'Cache-Control': 'public, s-maxage=600, max-age=60', // 10 mins on CDN, 1 min on clients
      'Cache-Tag': 'redirect, tag-redirect'
    })
    .redirect(
      302,
      createPackageURL(req.packageName, version, req.filename, req.search)
    );
}

function semverRedirect(req, res) {
  const maxVersion = semver.maxSatisfying(
    Object.keys(req.packageInfo.versions),
    req.packageVersion
  );

  if (maxVersion) {
    res
      .set({
        'Cache-Control': 'public, s-maxage=600, max-age=60', // 10 mins on CDN, 1 min on clients
        'Cache-Tag': 'redirect, semver-redirect'
      })
      .redirect(
        302,
        createPackageURL(req.packageName, maxVersion, req.filename, req.search)
      );
  } else {
    res
      .status(404)
      .type('text')
      .send(`Cannot find package ${req.packageSpec}`);
  }
}

function filenameRedirect(req, res) {
  let filename;
  if (req.query.module != null) {
    // See https://github.com/rollup/rollup/wiki/pkg.module
    filename = req.packageConfig.module || req.packageConfig['jsnext:main'];

    if (!filename) {
      // https://nodejs.org/api/esm.html#esm_code_package_json_code_code_type_code_field
      if (req.packageConfig.type === 'module') {
        // Use whatever is in pkg.main or index.js
        filename = req.packageConfig.main || '/index.js';
      } else if (
        req.packageConfig.main &&
        /\.mjs$/.test(req.packageConfig.main)
      ) {
        // Use .mjs file in pkg.main
        filename = req.packageConfig.main;
      }
    }

    if (!filename) {
      return res
        .status(404)
        .type('text')
        .send(`Package ${req.packageSpec} does not contain an ES module`);
    }
  } else if (
    req.query.main &&
    req.packageConfig[req.query.main] &&
    typeof req.packageConfig[req.query.main] === 'string'
  ) {
    // Deprecated, see #63
    filename = req.packageConfig[req.query.main];
  } else if (
    req.packageConfig.unpkg &&
    typeof req.packageConfig.unpkg === 'string'
  ) {
    filename = req.packageConfig.unpkg;
  } else if (
    req.packageConfig.browser &&
    typeof req.packageConfig.browser === 'string'
  ) {
    // Deprecated, see #63
    filename = req.packageConfig.browser;
  } else {
    filename = req.packageConfig.main || '/index.js';
  }

  // Redirect to the exact filename so relative imports
  // and URLs resolve correctly.
  res
    .set({
      'Cache-Control': 'public, max-age=31536000', // 1 year
      'Cache-Tag': 'redirect, filename-redirect'
    })
    .redirect(
      302,
      createPackageURL(
        req.packageName,
        req.packageVersion,
        addLeadingSlash(filename),
        createSearch(req.query)
      )
    );
}

/**
 * Fetch the package metadata and tarball from npm. Redirect to the exact
 * version if the request targets a tag or uses a semver version, or to the
 * exact filename if the request omits the filename.
 */
export default async function fetchPackage(req, res, next) {
  let packageInfo;
  try {
    packageInfo = await getNpmPackageInfo(req.packageName);
  } catch (error) {
    console.error(error);

    return res
      .status(500)
      .type('text')
      .send(`Cannot get info for package "${req.packageName}"`);
  }

  if (packageInfo == null || packageInfo.versions == null) {
    return res
      .status(404)
      .type('text')
      .send(`Cannot find package "${req.packageName}"`);
  }

  req.packageInfo = packageInfo;
  req.packageConfig = req.packageInfo.versions[req.packageVersion];

  if (!req.packageConfig) {
    // Redirect to a fully-resolved version.
    if (req.packageVersion in req.packageInfo['dist-tags']) {
      return tagRedirect(req, res);
    } else {
      return semverRedirect(req, res);
    }
  }

  if (!req.filename) {
    return filenameRedirect(req, res);
  }

  next();
}