$ node --v8-options | grep harm
 --harmony_typeof (enable harmony semantics for typeof)
 --harmony_scoping (enable harmony block scoping)
 --harmony_modules (enable harmony modules (implies block scoping))
 --harmony_symbols (enable harmony symbols (a.k.a. private names))
 --harmony_proxies (enable harmony proxies)
 --harmony_collections (enable harmony collections (sets, maps, and weak maps))
 --harmony_observation (enable harmony object observation (implies harmony collections)
 --harmony_typed_arrays (enable harmony typed arrays)
 --harmony_array_buffer (enable harmony array buffer)
 --harmony_generators (enable harmony generators)
 --harmony_iteration (enable harmony iteration (for-of))
 --harmony_numeric_literals (enable harmony numeric literals (0o77, 0b11))
 --harmony_strings (enable harmony string)
 --harmony_arrays (enable harmony arrays)
 --harmony (enable all harmony features (except typeof))

$ node --use-strict $(node --v8-options | grep harm | awk '{print $1}' | xargs) #ES6

node --harmony app.js

'use strict';
if (1) {
  let b = 2;
  console.log(b);  //2
}
console.log(typeof b); //undefined