https://ponyfoo.com/articles/es6-modules-in-depth
In case it isn’t immediately obvious – you should 'use strict' in all the places. Even though it’s becoming de-facto in ES6, it’s still a good practice to use 'use strict' everywhere in ES6.
In the ES6 module system, strict mode is turned on by default. In case you don’t know what strict mode is, it’s just a stricter version of the language that disallows lots of bad parts of the language. It enables compilers to perform better by disallowing non-sensical behavior in user code, too.
The following is a summary extracted from changes documented in the strict mode article on MDN.
- Variables can’t be left undeclared