How to work with private variables in ES6? [duplicate]

Answer 1

But in ES6, you can no longer declare vars outside the constructor

And you don't need to. You didn't do it in your ES5 constructor either. You can translate your code literally to

class Car {
    constructor() {
        // local variable
        var speed = 10;

        // public property
        this.model = "Batmobile";

        // public method
        this.init = () => {
            …
        }; // using an arrow function here simplifies things
    }
}