docusaurus/v2/lib/load/config.js

const fs = require('fs-extra');
const path = require('path');

module.exports = function loadConfig(siteDir, deleteCache = true) {
  const configPath = path.resolve(siteDir, 'siteConfig.js');
  if (deleteCache) {
    delete require.cache[configPath];
  }
  let config = {};
  if (fs.existsSync(configPath)) {
    config = require(configPath); // eslint-disable-line
  }

  const requiredFields = [
    'title',
    'tagline',
    'organizationName',
    'projectName',
    'baseUrl'
  ];
  const optionalFields = [
    'customDocsPath',
    'defaultLanguage',
    'highlight',
    'markdownPlugins',
    'configureWebpack',
    'chainWebpack',
    'docsUrl',
    'customFields'
  ];
  const missingFields = requiredFields.filter(field => !config[field]);
  if (missingFields && missingFields.length > 0) {
    throw new Error(
      `${missingFields.join(', ')} fields are missing in siteConfig.js`
    );
  }

  /* Fill default value */
  const defaultConfig = {
    customDocsPath: 'docs',
    docsUrl: 'docs'
  };
  Object.keys(defaultConfig).forEach(field => {
    if (!config[field]) {
      config[field] = defaultConfig[field];
    }
  });

  /* 
    User's own array of custom fields, 
    e.g: if they want to include some field so they can access it later from `props.siteConfig`
  */
  const {customFields = []} = config;

  /* We don't allow useless/ not meaningful field */
  const allowedFields = [...requiredFields, ...optionalFields, ...customFields];
  const uselessFields = Object.keys(config).filter(
    field => !allowedFields.includes(field)
  );
  if (uselessFields && uselessFields.length > 0) {
    throw new Error(
      `${uselessFields.join(', ')} fields are useless in siteConfig.js`
    );
  }

  return config;
};