read.js

import path from 'path';
import glob from 'globby';
import {readJson, outputJson} from 'fs-extra';

const cwd = process.cwd();

const read = (fp) => {
  return new Promise((res, rej) => {
    readJson(fp, (err, data) => {
      if (err) return rej(err);

      res(data);
    });
  });
};

const write = (data) => {
  return new Promise((res, rej) => {
    const dest = path.join(cwd, 'dist', 'combined.json');

    outputJson(dest, data, (err) => {
      if (err) return rej(err);

      res();
    });
  });
};

glob('mocks/*.json', {cwd}).then(fps => {
  const proms = fps.reduce((list, fp) => ([
    ...list,
    read(fp)
  ]), []);

  return Promise.all(proms);
}).then(data => {
  const combined = data.reduce((acc, json) => ({
    ...acc,
    ...json
  }), {});

  return write(combined);
}).catch(console.log.bind(console, '**ERR**'));