ES6/JavaScript - declaring variables without let/const

By omitting let, const, or var in non-strict mode, you create a property on the global object.

By the way, babel will add "use strict"; by default. So you will get a error with babel.

You can try it here: https://babeljs.io/repl/

In this case, x becomes a global variable. Try to avoid this as global variables can be hard on the browser.