docusaurus/v2/lib/load/docs/metadata.js

const fs = require('fs-extra');
const path = require('path');
const {getSubFolder, idx, parse} = require('../utils');

function getLanguage(filepath, refDir, env) {
  const translationEnabled = idx(env, ['translation', 'enabled']);
  if (translationEnabled) {
    const detectedLangTag = getSubFolder(filepath, refDir);
    const enabledLanguages = idx(env, ['translation', 'enabledLanguages']);
    const langTags =
      (enabledLanguages && enabledLanguages.map(lang => lang.tag)) || [];
    if (langTags.includes(detectedLangTag)) {
      return detectedLangTag;
    }
    const defaultLanguage = idx(env, ['translation', 'defaultLanguage']);
    if (defaultLanguage && defaultLanguage.tag) {
      return defaultLanguage.tag;
    }
  }
  return undefined;
}

function getVersion(filepath, refDir, env) {
  const versioningEnabled = idx(env, ['versioning', 'enabled']);
  if (versioningEnabled) {
    const subFolder = getSubFolder(filepath, refDir);
    if (subFolder) {
      const detectedVersion = subFolder.replace(/^version-/, '');
      const versions = idx(env, ['versioning', 'versions']) || [];
      if (versions.includes(detectedVersion)) {
        return detectedVersion;
      }
    }
    return 'next';
  }
  return undefined;
}

module.exports = async function processMetadata(
  source,
  refDir,
  env,
  order,
  siteConfig,
) {
  const filepath = path.resolve(refDir, source);
  const fileString = await fs.readFile(filepath, 'utf-8');
  const {metadata} = parse(fileString);

  /* default id is the file name */
  if (!metadata.id) {
    metadata.id = path.basename(source, path.extname(source));
  }
  if (metadata.id.includes('/')) {
    throw new Error('Document id cannot include "/".');
  }

  /* default title is the id */
  if (!metadata.title) {
    metadata.title = metadata.id;
  }

  /* language */
  const language = getLanguage(filepath, refDir, env);
  metadata.language = language;
  const langPart = (language && `${language}/`) || '';

  /* version */
  const defaultLangTag = idx(env, ['translation', 'defaultLanguage', 'tag']);
  let versionRefDir = refDir;
  if (language && language !== defaultLangTag) {
    versionRefDir = path.join(refDir, language);
  }
  const version = getVersion(filepath, versionRefDir, env);
  metadata.version = version;
  const latestVersion = idx(env, ['versioning', 'latestVersion']);
  const versionPart =
    (version && version !== latestVersion && `${version}/`) || '';

  /* 
    Convert temporarily metadata.id to the form of dirname/id without version/lang prefix
    ex: file `versioned_docs/version-1.0.0/en/foo/bar.md` with id `version-1.0.0-bar` => `foo/bar`
  */
  if (language) {
    metadata.id = metadata.id.replace(new RegExp(`^${language}-`), '');
  }

  if (version) {
    metadata.id = metadata.id.replace(new RegExp(`^version-${version}-`), '');
  }

  const dirName = path.dirname(source);
  if (dirName !== '.') {
    let prefix = dirName;
    if (language) {
      prefix = prefix.replace(new RegExp(`^${language}`), '');
    }
    prefix = prefix.replace(/^\//, '');
    if (version) {
      prefix = prefix.replace(new RegExp(`^version-${version}`), '');
    }
    prefix = prefix.replace(/^\//, '');
    if (prefix) {
      metadata.id = `${prefix}/${metadata.id}`;
    }
  }

  /* 
    The docs file source
    e.g: `@docs/hello.md` or `@versioned_docs/version-1.0.0/hello.md` 
  */
  if (language && language !== defaultLangTag) {
    metadata.source = `@translated_docs/${source}`;
  } else if (version && version !== 'next') {
    metadata.source = `@versioned_docs/${source}`;
  } else {
    metadata.source = `@docs/${source}`;
  }

  /* Build the permalink */
  const {baseUrl, docsUrl} = siteConfig;

  /* 
    if user has own custom permalink defined in frontmatter
    e.g: :baseUrl:docsUrl/:langPart/:versionPart/endiliey/:id
  */
  if (metadata.permalink) {
    metadata.permalink = path.resolve(
      metadata.permalink
        .replace(/:baseUrl/, baseUrl)
        .replace(/:docsUrl/, docsUrl)
        .replace(/:langPart/, langPart)
        .replace(/:versionPart/, versionPart)
        .replace(/:id/, metadata.id),
    );
  } else {
    metadata.permalink = `${baseUrl}${docsUrl}/${langPart}${versionPart}${
      metadata.id
    }`;
  }

  /* if version */
  if (version && version !== 'next') {
    metadata.id = `version-${version}-${metadata.id}`;
  }

  /* save localized id before adding language on it */
  metadata.localized_id = metadata.id;

  /* if language */
  if (language) {
    metadata.id = `${language}-${metadata.id}`;
  }

  /* Determine order */
  const id = metadata.localized_id;
  if (order[id]) {
    metadata.sidebar = order[id].sidebar;
    metadata.category = order[id].category;
    if (order[id].next) {
      metadata.next_id = order[id].next;
      metadata.next = (language ? `${language}-` : '') + order[id].next;
    }
    if (order[id].previous) {
      metadata.previous_id = order[id].previous;
      metadata.previous = (language ? `${language}-` : '') + order[id].previous;
    }
  }
  return metadata;
};