Catching Unresolved Vars

const vm = require('vm')
const repl = require('repl')

const globals = {
  Date,
  JSON
}

const state = {}

const environment = new Proxy(state, {
  get (target, key) {
    if (key === 'context') return state

    return globals[key] || target[key]
  },

  set (target, key, value) {
    target[key] = value
  }
})

const ctx = vm.createContext(environment)

repl.start({
  prompt: '> ',
  eval: (command, context, filename, cb) =>
    Promise.resolve(vm.runInContext(command, ctx)).then(result => cb(null, result))
})