$ 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