Scan + merge except for defaultLng, with support for removeUnusedKeys option

i18next-scanner.config.js

/* eslint-disable import/no-extraneous-dependencies */
const _ = require('lodash');
const fs = require('fs');
const eol = require('eol');
const path = require('path');
const VirtualFile = require('vinyl');
const flattenObjectKeys = require('i18next-scanner/lib/flatten-object-keys')
  .default;
const omitEmptyObject = require('i18next-scanner/lib/omit-empty-object')
  .default;

function getFileJSON(resPath) {
  try {
    return JSON.parse(
      fs
        .readFileSync(fs.realpathSync(path.join('src', resPath)))
        .toString('utf-8'),
    );
  } catch (e) {
    return {};
  }
}

function flush(done) {
  const { parser } = this;
  const { options } = parser;

  // Flush to resource store
  const resStore = parser.get({ sort: options.sort });
  const { jsonIndent } = options.resource;
  const lineEnding = String(options.resource.lineEnding).toLowerCase();

  Object.keys(resStore).forEach(lng => {
    const namespaces = resStore[lng];

    Object.keys(namespaces).forEach(ns => {
      let obj = namespaces[ns];

      const resPath = parser.formatResourceSavePath(lng, ns);

      // if not defaultLng then Get, Merge & removeUnusedKeys of old JSON content
      if (lng !== options.defaultLng) {
        let resContent = getFileJSON(resPath);

        if (options.removeUnusedKeys) {
          const namespaceKeys = flattenObjectKeys(obj);
          const resContentKeys = flattenObjectKeys(resContent);
          const unusedKeys = _.differenceWith(
            resContentKeys,
            namespaceKeys,
            _.isEqual,
          );

          for (let i = 0; i < unusedKeys.length; ++i) {
            _.unset(resContent, unusedKeys[i]);
          }

          resContent = omitEmptyObject(resContent);
        }

        obj = { ...obj, ...resContent };
      }

      let text = `${JSON.stringify(obj, null, jsonIndent)}\n`;

      if (lineEnding === 'auto') {
        text = eol.auto(text);
      } else if (lineEnding === '\r\n' || lineEnding === 'crlf') {
        text = eol.crlf(text);
      } else if (lineEnding === '\n' || lineEnding === 'lf') {
        text = eol.lf(text);
      } else if (lineEnding === '\r' || lineEnding === 'cr') {
        text = eol.cr(text);
      } else {
        // Defaults to LF
        text = eol.lf(text);
      }

      this.push(
        new VirtualFile({
          path: resPath,
          contents: Buffer.from(text),
        }),
      );
    });
  });

  done();
}

module.exports = {
  input: [
    'src/**/*.{js,jsx}',
    // Use ! to filter out files or directories
    '!src/**/*.spec.{js,jsx}',
    '!src/i18n/**',
    '!**/node_modules/**',
  ],
  output: './src',
  options: {
    debug: false,
    removeUnusedKeys: true,
    sort: true,
    func: {
      list: ['i18next.t', 'i18n.t', 't'],
      extensions: ['.js', '.jsx'],
    },
    lngs: ['en', 'fr'],
    defaultLng: 'en',
    keySeparator: false,
    resource: {
      loadPath: 'i18n/locales/{{lng}}.json',
      savePath: 'i18n/locales/{{lng}}.json',
    },
  },
  flush,
};

Great! Thanks!