R = Immutable.Record({
  foo: undefined,
  bar: undefined
})
r0 = new R({
  foo: 'poo',
  bar: 'BAR'
})
r1 = new R({
  foo: 'FOO'
})
console.log(r1.toJS()) // Object {foo: "FOO", bar: undefined}
r2 = r1.map(function(value, key) {
  return value || r0.get(key)
})
console.log(r2) // undefined
console.log(r1.toJS()) // Object {foo: "FOO", bar: "BAR"}