code to join some JSON files into one Object..

index.js

var fs = require('fs')
  , extend = require('extend')
  , async = require('async')
  , obj = {}

var files = [
    '001-050.json'
  , '051-100.json'
  , '101-150.json'
  , '151-200.json'
]

async.map(files, fs.readFile, function(err, results) {
  // Note: results contains an Array of Buffers
  results.forEach(function(item) {
    obj = extend(obj, JSON.parse(item.toString('utf8')))
  })
  console.log(Object.keys(obj), 'count: '+ Object.keys(obj).length)
})