json_transform.js

const get = require('lodash/get');

function transform(source, schema, ...ext) {
  const result = Object.create(null);
  for (let prop of Object.getOwnPropertyNames(schema)) {
    if (typeof schema[prop] === 'function') {
      result[prop] = schema[prop](source[prop], source);
    } else {
      result[prop] = schema[prop];
    }
  }
  return Object.assign({}, ...ext, result);
}


const source = {
  name: 'name',
  users: [
    {
      name: 'user1',
      age: 28
    }, {
      name: 'user2',
      age: 12
    }
  ],
  index: 1,
  meta: {
    error: null,
    status: 'ok'
  }
};

const schema = {
  status: (_, source) => get(source, 'meta.status'),
  name: value => `${value} modified`,
  users: function(value, source) {
    return value.map(v => ({
      name: get(v, 'name'),
      age: get(v, 'age'),
      status: get(source, 'meta.status'),
    }));
  },
};

console.log(transform(source, schema, source))
console.log(transform(source, schema, source, {ext: 'v1', ext2: ['v2']}))